UNIVERSITY  OF  CALIFORNIA 
AT    LOS  ANGELES 


NATURAL    PHILOSOPHY. 


NATURAL  PHILOSOPHY 


COMMON   AND   HIGH    SCHOOLS. 


LE  EOT  O.   COOLET,   PH.  D., 

PROFES808   OP   NATUBAL   SCIENCE   IN   TUB   MEW    YOUK    STATE    NORMAL    SCHOOL. 


NEW  YORK: 
SCRIBNER,   ARMSTRONG  &  CO., 

SUCCESSORS  TO 

CHARLES  SCRIBNER  &  CO., 

654    BROADWAY. 
1872. 


Entered  according  to  Act  of  Congress,  in  the  year  1871, 

BY  LK  KOY  C.  COOLKY, 
lu  the  Office  of  the  Librarian  of   Coiigress,  at   Washington 


CT] 


CO 

oo 

CD 


PREFACE 


THE  great  aim  of  this  little  book  is  to  present  the  most 
^elementary  facts  of  Natural  Philosophy,  iu  such  a  way  as  to 
Sexercise  the  child  constantly  in  observing  phenomena  and  in 
£3 drawing  inferences  from  what  he  observes. 
3  "Whenever  a  child  is  old  enough  to  ask  such  questions,  as, 
"  What  makes  the  thunder,"  or,  "Where  does  the  rain  come 

(»J from,"  or  to  exclaim   "  How  pretty  the  clouds  are  this  even- 

2 . 

~~  ing,"  it  is  old  enough  to  begin  the  study  of  natural  philosophy. 

OO 

,-.  When  snch  questions  are  asked  the  mind  is  awake  to  see  the 

^phenomena   of  nature,  and  "is  ready  to   receive   instruction. 

**  They  show  the  presence  of  a  desire  to  know,  and  the  absence 

of  power  to  learn  without  assistance ;  and  in  this  way  they 

£jlead  us  to  believe  that  the  time  has  cohae  when  the  work  of 

minstruction  should  begin. 

^g 

C     Moreover,  the  study  of  natural  philosophy  is  easy,  and  in- 

Cteresting  to  young  pupils,  because,  when  properly  presented, 

°it   brings    ne\v  sights  to  the  eye,  and  new  sounds  the  ear, 

in  a  way  to  be  especially  pleasant  to  children.     The  simplest 

experiments  awaken  enthusiasm  in  the  mind  of  a  child,  and 

such  as  he  may  be  able  to  repeat  by  himself  are  the  source 

of  the  greatest  delight. 


233560 


6  PREFACE. 

This  study  is  not  only  easy  and  interesting,  it  is  also  in  the 
highest  degree  beneficial  to  the  young,  partly  because  of  the 
valuable  facts  it  imparts,  but  even  more  on  account  of  the 
mental  power  it  devclopes.  The  object  of  primary  education 
should  be  to  discipline  the  senses  to  habits  of  quick  and  ac- 
curate observation,  and  the  mind  to  the  habit  of  forming 
correct  judgments  from  facts  which  the  senses  reveal. 
Natural  Philosophy  furnishes  abundant  materials  of  the  most 
excellent  kind,  by  means  of  which  these  objects  may  be  ac- 
complished. There  are  curious  motions,  beautiful  colors  nnd 
harmonious  sounds,  together  with  numerous  other  phenom- 
ena, which  can  be  easily  presented  in  the  form  of  simple  ex- 
periments, by  which  the  skilful  teacher  can  cultivate  the 
power  of  the  senses  to  furnish  correct  impressions,  and  at 
the  same  time  develope  the  power  of  basing  accurate  judg- 
ments upon  the  impressions  received.  In  a  word,  this  study 
when  properly  presented  is  eminently  fitted  to  teach  even 
young  pupils  how  to  gain  knowledge  for  themselves  by  ob- 
serving events. 

To  this  end,  the  following  plan  ought  to  prevail  in  present- 
ing elementary  facts.  •  An  easy  experiment  or  some  phenom- 
enon of  common  occurrence,  is  to  be  introduced  and  the  atten- 
tion of  the  child  directed  to  certain  appearances  and  condit- 
ions, after  which  he  may  be  called  upon  to  notice  the  truth 
which  these  appearances  suggest.  A  concise  and  accurate 
statement  of  the  fact  or  principle  itself,  in  form  to  be  easily 
remembered,  may  finally  complete  the  investigation. 

Many  of  the  experiments  are  such  as  pupils  can  make  for 
themselves:  let  them  be  encour.iged  to  do  so;  if  they  are, 


PREFACE.  7 

they  will  soon  be  bringing  to  the  notice  of  the  teacher, 
others  which  the  text  does  not  describe,  and  if  the  teacher 
will  visit  such  efforts  with  marks  of  especial  notice  or  reward 
ho  will  soon  find  an  enthusiasm  in  his  class,  which  will 
make  the  pursuit  of  this  study  delightful  and  profitable  to 
the  end. 

Another  feature  of  this  little  book,  which,  it  is  believed, 
will  commend  it  to  the  favor  of  both  pupil  and  teacher,  is  the 
system  of  questions  which  runs  through  it.  Every  impor- 
tant topic  in  the  discussion  of  each  subject  is  introduced  by 
a  question,  instead  of  by  a  formal  title,  as  is  customary. 
These  questions  will  prove  to  be  excellent  guides,  and  really 
very  important  helps  to  the  young  pupil.  The  teacher  will 
also  find  them  serviceable  in  conducting  the  exercises  of  the 
class  room,  to  which  they  are  especially  adapted,  by  being  in 
immediate  connection  with  the  text,  and  in  bold  type  easily 
'caught  by  the  eye,  instead  of  at  the  bottom  of  the  page  or  the 
back  of  the  book,  in  fine  print  and  compact  lists.  The  eye 
catching  them  quickly,  is  not  confined  to  the  book  ;  their 
use,  therefore,  will  not  be  at  the  sacrifice  of  the  vivacity  and 
vigor  of  the  exercise. 

Albany,  1871. 


NATURAL    PHILOSOPHY. 


PROPERTIES   OF  MATTER 


Describe  the  experiment  with  cochineal. — If,  to 

try  an  easy  experiment,  we  take  a  single  grain  weight  of 
cochineal,  and  dissolve  it  in  as  much  as  a  thimbleful  of 
water,  and  then  pour  this  small  quantity  into  a  gallon  of 
clear  water,  the  whole  gallon  will  receive  a  beautiful  crim- 
son color. 

Into  how  many  pieces  has  the  grain  of  cochi- 
neal been  divided  ? — Now  a  gallon  of  water  is  said  to 
contain  as  many  as  60,000  drops,  and  to  color  a  single 
drop,  all  through,  will  take  as  many  as  100  little  particles 
of  cochineal.  If  this  is  true,  then  the  grain  of  cochineal 
must  be  divided  into  not  less  than  six  millions  of  pieces  ! 

Can  other  bodies  be  divided  ? — If  an  apple  be  cut 
into  100  pieces,  each  piece  will  of  course  be  very  small 
indeed,  but  yet  it  will  not  be  BO  small  that  it  can  not  be 
divided  into  pieces  smaller  yet. 

The  blow  of  a  hammer  may  break  a  pane  of  glass  into 
a  thousand  parts,  but  each  one  of  these  little  pieces  may 
by  another  blow  be  broken  into  pieces  still  smaller. 
i* 


10  NATURAL  PHILOSOPHY. 

What  is  divisibility  ? — Every  body  of  matter  may 
be  cut  or  broken  into  pieces.  This  is  one  of  the  qualities 
or  properties  of  matter,  and  we  call  it  divisibility. 

Divisibility  is  the  property  of  matter  in  virtue  of 
which  a  body  may  be  separated  into  parts. 

Are  examples  of  great  divisibility  common  ? 
— There  are  bodies  all  around  us  so  small  that  we  can  not 
see  them.  They  are  in  the  air  we  breathe  and  in  the  water 
we  drink.  Some  of  them  are  alive  and  some  are  not. 
Many  of  them  are  so  very  small  that  we  need  the  most 
powerful  microscope  to  see  them  at  all.  Yet  every  one  is 
made  up  of  pieces  or  parts  which  are  of  course  smaller 
than  itself. 

For  example :  the  dust  which  clings  to  one's  finger 
when  he  holds  a  butterfly  or  a  moth  is  made  up  of 
very  small  particles,  and  yet  each  of  these  little  particles 
of  dust,  which  we  can  scarcely  see  with  the  naked  eye,  is 
found,  by  using  the  microscope,  to  be  made  up  of  a 
thousand  or  more  little  balls. 

Are  living  creatures  so  very  small? — And  then, 
too,  there  are  living  creatures  so  small,  that  it  may  need  as 
many  as  a  million  of  them  to  make  a  pile  as  large  as  a 
mustard-seed.  Hosts  of  them  are  living  in  the  air  and  in 
the  water  all  around  us.  They  are  so  very  very  smali 
that  it  has  been  said  that  a  thousand  of  them  might  sWim 
or  fly  sid?  by  side  through  the  eye  of  a  needle. 

And  yet  each  of  these  little  creatures  must  be  made  up 
of  still  smaller  parts,  or  else  they  could  not  move  about 
nor  devour  their  food,  as  all  of  them  are  able  to  do.  We 
can  not  even  imagine  how  very  small  these  parts  must 
be. 

What  are  molecules  ?— If  we  keep  on  dividing  a 
"oody  into  smaller  and  smaller  pieces,  we  shall  at  last  get 


PROPERTIES  OF   MATTER.  H 

to  a  piece  so  very  small  that  it  can  not  be  divided  again 
without  changing  it  into  some  other  kind  of  matter. 
These  smallest  pieces  are  called  molecules. 

Molecules  are  particles  of  matter  which  can  not  be 
divided  without  changing  their  nature.  -^ 

Does  every  body  occupy  space  ? — Every  little  par- 
ticle of  dust,  and  even  a  molecule,  must  have  some  size. 
You  can  not  even  think  of  a  body  which  should  have  no 
size  at  all.  The  very  smallest  body  you  can  think  of  fills 
up  a  little  room  or  space.  And  then  every  larger  thing,  a 
%hot  for  example,  or  a  cannon-ball ;  the  worl<J  itself,  so 
many  millions  of  times  larger  than  the  ball ;  and  the  sun, 
which  is  fourteen  hundred  thousand  times  larger  than  the 
world, — each  of  these  bodies  has  its  own  particular  size. 
or,  in  other  words,  each  one  fills  a  certain  portion  of 
space. 

To  occupy  space  is  one  of  the  properties  of  all  matter : 
it  is  called  extension. 

Extension  is  the  property  of  matter  in  virtue  of  which 
a  body  occupies  a  certain  portion  of  space. 

Can  wood  and  -water  fill  the  same  place  at  once  ? 

— If  we  fill  a  goblet  with  water,  and  then  gently  push  a 
small  stick  down  into  it,  the  water  runs  over.  We  see 
in  this  way  that  wood  and  water  can  not  be  put  into  the 
same  place  at  the  same  time. 

Can  water  and  air  fill  the  same  place  at  once  ? 
— Boys  sometimes  turn  a  goblet  bottom  upward,  and  then 
push  it  down  into  a  vessel  of  water  to  see  the  air  keep  the 
water  out  of  the  goblet.  The  air  will  not  let  the  water 
in,  because  they  can  not  both  be  in  the  same  place  at 
once. 


12  NATURAL  PHILOSOPHY. 

Can  not  a  nail  be  driven  into  the  same  place 
with  wood  ? — When  a  nail  is  driven  into  wood,  the  par- 
ticles of  the  wood  are  squeezed  nearer  together  so  as  to 
make  room  for  the  nail  to  enter.  The  wood  and  the  nail 
do  not  fill  the  same  space  at  the  same  time. 

What  is  impenetrability? — No  two  bodies  can 
ever  be  in  exactly  the  same  place  at  once.  This  is 
one  of  the  properties  of  matter.  It  is  called  impenetra- 


Impenetrability  is  the  property  of  matter  which  does 
not  allow  two  bodies  to  occupy  the  same  space  at  the  same 
time. 

Can  not  water  and  sugar  be  put  into  the  same 
space  at  once? — Try  an  easy  experiment  in  this  way: 
till  a  goblet  to  the  very  brim  with  water,  so  that  not  a 
drop  more  can  be  added  without  running  over.  Then 
take  some  fine  sugar,  and  very  slowly  sprinkle  it  into 
the  water.  Quite  a  large  quantity  may  be  put  into  the 
goblet  before  a  single  drop  of  water  will  overflow.  It 
would  at  first  appear  that  water  and  sugar  have  been  here 
put  into  the  same  space  at  the  same  time. 

Describe  the  experiment  with  shot.  —  Take 
another  goblet  and  fill  it  with  shot,  and  then  pour  fine 
sand  upon  the  shot.  The  sand  will,  of  course,  fall  down 
between  the  shot,  and  a  large  quantity  will  be  poured 
in  before  any  will  run  over.  Now  the  sand  only  fills  up 
the  space  between  the  shot ;  no  one  would  think  of  saying 
that  the  sand  and  shot  fill  the  same  space  at  once. 

How  does  this  explain  the  experiment  with 
sugar  and  water? — Now  the  sugar  and  water  act  in 
the  same  way  as  the  sand  and  sHot.  The  fact  is  that 
water  is  made  up  of  little  balls  01  molecules,  and  the 


PROPERTIES   OF   MATTER.  13 

molecules  of  sugar  are  little  enough  to  fall  into  the  spaces 
between  them.  The  two  things  do  not  occupy  the  same 
space  at  once. 

What  should  we  learn  from  the  experiment? — 
We  see  that  there  must  be  spaces  between  the  molecules 
ot  water  into  which  the  sugar  falls.  It  is  true  also  of  all 
other  bodies  that  there  are  spaces  between  their  mole- 
cules. On  this  account  bodies  are  said  to  be  porous. 

Porosity  is  the  property  of  matter  in  virtue  of  which 
there  are  spaces  between  its  molecules. 

How  was  gold  shown  to  be  porous  ? — A  long  time 
ago,  at  Florence,  a  hollow  globe  of  gold  was  filled  with 
water,  shut  up  perfectly  tight,  and  then  put  under  immense 
pressure!  The  water  actually  oozed  through  the  gold, 
and,  like  a  gentle  dew,  covered  the  outside.  The  water 
must  have  come  through  between  the  molecules  of  the 
gold,  so  that  we  know  that  even  gold  is  porous. 

What  is  elasticity  ?— We  all  knoAv  that  a  piece  of 
india-rubber  can  be  pulled  out  to  a  great  length,  and  that 
it  will  afterward  spring  back  again.  We  have  also  doubt- 
less seen  a  steel  wire  straighten  itself  quickly  after  being 
bent.  The  quality  or  property  which  causes  these  bodies 
to  spring  back  is  called  elasticity. 

Elasticity  is  the  property  of  matter  in  virtue  of 
which  a  body  springs  lack  after  having  yielded  to  some 
force. 

Is  glass  elastic  ? — The  balls  which  boys  use  to  play 
"  marbles  "  writh  are  sometimes  made  of  glass,  and  every 
one  knows  how  well  one  of  these  balls  will  bound  upward 
when  thrown  upon  the  floor  or  pavement.  Now  the  little 
ball  is  actually  flattened  at  the  moment  when  it  strikes 
the  floor,  but  the  next  instant  the  flattened  part  springs 


14  NATURAL  PHILOSOPHY. 

back,  and  it  is  this  springing  back  which  throws  the  ball 
into  the  air.  This  shows  that  glass  is  very  elastic. 

Are  all  bodies  elastic  ? — All  bodies  are  more  or  less 
elastic.  Lead  and  clay  have  but  little  elasticity,  but  even 
"lead  is  elastic,  for  we  nnd  that  two  lead  balls  after  being 
struck  together  w:ll  bound  a  little. 

On  the  other  hand,  glass  and  ivory  are  among  the  most 
elastic  solids  we  know  of. 

What  becomes  of  water  that  boils  away  ? — When 
water  boils,  steam  is  formed.  Now  let  us  prove  that  all 
the  water  which  disappears  is  changed  into  steam. 

."We  will  first  notice  that  when  a  cold  body  is  held  in 
a  cloud  of  steam  it  is  soon  wet,  which  shows  that  by  cool- 
ing the  steam  we  get  back  water  from  which  it  was  made. 

Now  let  us  put  just  one  pound  of  water  into  a  vessel 
and  boil  it.  Let  the  steam  pass  through  a  pipe  into 
another  vessel  which  is  kept  cold.  The  steam  goes  over 
into  the  cold  vessel,  and  is  changed  back  into  water. 
When  the  water  is  all  "  boiled  away "  from  the  first 
vessel  we  shall  find  just  one  pound  of  water  in  the  other. 
All  the  water  which  disappeared  was  changed  into  steam. 
Not  a  particle  was  destroyed. 

Is  this  true  when  the  steam  goes  into  the  air  ? — 
When  the  steam  goes  oft'  into  the  air,  as  it  usually  does, 
its  particles  are  so  widely  scattered  that  they  disappear 
entirely,  but  every  one  of  them  is  still  in  existence  some- 
where. 

What  becomes  of  wood  when  it  burns  ? — When 
wood  is  burned,  one  part  of  it  is  changed  into  ashes,  and 
the  rest  changes  into  smoke  and  vapor.  Not  a  single 
particle  of  it  is  ever  destroyed. 

What  is  indestructibility  ? — Man  may  thus  change 


PROPERTIES  OF  MATTER.  15 

the  shape  and  condition  of  bodies,  but  he  can  not  destroy 
a  single  molecule  of  any  thing.  On  this  account  matter 
is  said  to  be  indestructible. 

Indestructibility  is  the  property  of  matter  in  virtue  of 
which  it  can  not  be  destroyed. 

What  is  compressibility? — When  we  squeeze  a 
sponge  in  the  hand  we  press  its  particles  nearer  and 
nearer  together,  and  finally  make  the  sponge  much 
smaller  than  it  was.  Bodies  which  can  be  made  smaller 
by  pressure  are  said  to  be  compressible. 

Compressibility  is  the  property  of  matter  in  virtue  of 
which  a  body  can  be  made  smaller  l>y  pressure. 

Is  air  compressible  ? — Air  is  very  compressible.  You 
may  learn  this  from  so  humble  a  thing  as  a  pop-gun. 
Before  the  stopper  is  blown  out,  the  air  behind  it  you 
can  see  to  be  crowded  into  perhaps  not  half  the  space  it 
filled  at  first. 

Is  water  compressible  ? — Water  is  very  slightly 
compressible.  Very  great  pressure  is  needed  to  compress 
it  enough  to  show  that  it  is  compressible  at  all.  A  press- 
ure which  would  compress  air  into  less  than  a  hundredth 
part  of  its  natural  bulk  would  not  compress  water,  enough 
to  be  noticed. 

Are  all  bodies  compressible  ? — All  bodies  are  more 
or  less  compressible.  Air  is  one  of  the  most  compressi- 
ble of  all  substances,  and  water  is  one  which  is  among 
the  least  compressible. 

Are  molecules  compressible  ? — All  that  pressure 
can  do  to  a  body  is  to  push  its  molecules  nearer  together : 
we  do  not  suppose  that  it  makes  the  molecules  themselves 
any  smaller. 

When  air  is  compressed  into  a  hundredth  part  of  its 


]  tf  NATURAL  PHILOSOPHY. 

natural  volume,  its  molecules  have  been  pushed  a  hundred 
times  nearer  together  than  they  were  at  first. 

What  is  density  ?— Now  when  the  air  is  so  much  com- 
pressed there  is  much  more  of  it  in  a  given  space,  a  cubic 
inch  for  example,  than  there  was  before.  In  this  con- 
dition it  would  be  said  to  be  more  dense. 

Density  has  reference  to  the  quantity  of  matter  in  a, 
given  bulk. 

What  is  inertia  ? — Masses  of  matter  have  no  power 
to  move  themselves  nor  to  stop  themselves  when  once  in 
motion.  The  clouds  move  along  in  the  sky,  not  because 
they  choose  to  do  so,  but  because  they  are  pushed  along 
by  the  wind.  An  apple  falls  from  the  tree,  because  it  is 
pulled  down  by  an  influence,  soon  to  be  described,  called 
gravitation  ;  and  rocks  rest  in  their  places,  not  because 
they  have  any  power  in  themselves  to  do  so,  but  because 
they  are  held  there  by  forces  acting  upon  them.  Bodies 
of  matter  have  no  power  to  change  their  own  condition, 
and  on  this  account  they  are  said  to  be  inert.  Inertia  in 
the  property  of  matter  which  does  not  allow  a  body  to 
change  its  own  condition  of  rest  or  motion. 

Are  molecules  ever  at  rest?— Masses  of  matter 
are  often  at  rest ;  it  is  believed  that  molecules  never  are. 
On  the  contrary,  it  is  thought  that  the  molecules  of  every 
body  are  forever  in  motion. 

You  have  perhaps  seen  a  cluster  of  bees  at  the  door  of 
their  hive,  or  of  ants  at  the  entrance  to  their  nest,  all  hud- 
dled together  and  hurrying  over  and  around  each  other  in 
constant  and  curious  motions.  Now  if  our  eyes  were  power- 
ful enough  to  see  the  little  molecules  of  which  a  block  of 
wood  is  composed,  it  is  thought  that  we  should  witness  a 


PROPERTIES  OF  MATTER.  17 

scene  of  activity  still  more  curious  and  constant,  for  every 
molecule  in  all  the  vast  number  which  the  block  contains 
is  in  rapid  motion.  Philosophers  believe  that  not  a  single 
one  in  all  the  world  was  ever  for  a  moment  still. 

In  -what  respect  are  the  properties  so  far  de- 
scribed, alike?— If  we  think  again  of  the  properties 
which  we  have  just  examined,  we  find  that  they  are  all 
of  such  a  character  tnat  a  body  may  show  that  it  has 
them  without  any  change  taking  place  in  its  nature. 

A  smart  blow  with  a  hammer  shatters  a  stone  into 
fragments,  and  the  experiment  teaches  us  that  the  stone 
lias  the  property  of  divisibility.  But  then-  every  piece 
will  be  a  piece  of  stone,  and  of  just  the  same  kind  as  be- 
fore the  blow  ;  and  so  we  find  that  a  body  may  show  the 
property  of  divisibility  without  any  change  in  its  nature. 

What  are  physical  properties  ?— All  such  proper- 
ties,— that  is,  all  properties  which  a  body  may  show  with- 
out any  change  in  its  nature, — are  called  physical  proper- 
ties. 

What  are  chemical  properties  ?— But  all  proper- 
ties are  not  like  these.  Explosibility,  for  example,  is  one 
which  a  body  can  not  show  without  a  change  in  its  nature. 

Suppose  a  little  gunpowder  lies  upon  the  table.  You 
do  not  know  whether  it  is  explosive  or  not :  it  may  be 
damp,  and  hence  not  explosive.  Bui  you  touch  it  with  a 
lighted  match — a  bright  flash  and  a  sudden  puff  occurs, 
and  you  say  that  the  powder  is  explosive.  Now  all  that 
is  left  upon  the  table  at  the  spot  is  a  dark  stain.  The 
powder  itself  has  been  changed  into  gases  which  have 
passed  off  and  hidden  in  the  air.  No  body  of  matter  can 
show  that  it  has  the  property  of  explosibility  without 
changing  to  something  else,  and  for  this  reason  explosibil- 


13  NATURAL  PHILOSOPHY. 

ity  is  called  a  chemical  property.  Chemical  properties 
are  those  which  a  body  can  not  show  without  a  change  in 
its  nature. 

What  is  natural  philosophy? — Natural  philosophy 
is  the  science  which  treats  of  the  physical  properties  of 
matter  and  explains  those  things  which  occur  without 
any  change  in  the  nature  of  bodies. 

The  chemical  properties  of  matter  are  to  be  described 
in  the  science  of  chemistry  :  we  need  give  no  further 
attention  to  them  now. 


ATTRACTION. 


What  is  the  effect  of  rubbing  sealing-wax  -with 
flannel? — If  we  briskly  rub  a  stick  of  sealing-wax  with  a 
piece  of  flannel  or  silk,  we  seem  to  give  it  a  power  which 
it  did  not  have  before,  for  if  we  hold  it  near  to  small  bits 

Fig.  1. 


of  cotton  we  see  them  fly  quickly  toward  it,  or  if  we  pre- 
sent it  to  a  pith-ball  hung  by  a  silk  thread,  the  ball  will 
be  drawn  aside  or  lifted  by  it.  (Fig.  1.) 

What  may  be  seen  on  the  surface  of  quiet  water  ? 
— If  we  observe  the  surface  of  quiet  pools  of  water,  we 


20  NATURAL  PHILOSOPHY. 

notice  that  sticks  and  straws  will  not  stay  for  any  great 
length  of  time  upon  the  middle  parts  of  the  surface,  but 
that,  instead  of  this,  they  will  be  gathered  together  around 
the  edges. 

Or,  if  we  wish  to  try  an'  experiment,  we  may  put  a 
number  of  bits  of  wood  here  and  there  upon  the  surface  of 
water  in  a  large  pail  or  tub  standing  in  a  quiet  place 
where  it  may  rest  over  night.  In  the  morning  we  will 
find  the  bits  of  wood  huddled  together,  or  against  the  side 
of  the  vessel :  not  one  of  them  staying  alone  where  it  was 
placed. 

What  other  facts  of  the  same  kind  ?— Other  facts 
of  the  same  kind  are  still  more  familiar.  A  stone  moves 
toward  the  ground  when  'not  supported.  Leaves  fall  to 
the  earth  in  autumn,  and  rain-drops  and  hail  stones  will 
not  abide  in  the  sky. 

What  do  these  experiments  and  facts  illustrate  ? 
— Now  all  these  experiments  and  facts  illustrate  the  ten- 
dency of  all  bodies  of  matter  to  approach  each  other.  If 
they  were  not  kept  apart  by  some  other  forces  this 
tendency  would  cause  all  bodies  to  rush  together.  The 
influence  that  would  bring  them  together  is  called  attrac- 
tion. 

Name  varieties  of  attraction. — Attraction  shows 
itself  in  many  ways,  and  when  acting  in  different  ways  it 
is  called  by  different  names.  When  magnets  attract  each 
other  the  influence  is  called  magnetic  attraction.  The 
influence  of  the  sealing-wax  upon  the  pith-ball  (Fig.  1)  is 
called  electrical  attraction.  Besides  these  there  are  other 
varieties,  called  cohesion,  adhesion,  and  iOMiiation. 

With  the  last  three  varieties  we  must  now  become 
acquainted  ;  but  of  the  first  two  we  shall  learn  more  at 
another  time. 


ATTRACTION.  21 

Why  is  a  rod  of  iron  so  strong  ?— It  is  by  no  means 
easy  to  break  a  rod  of  iron.  Every  child  knows  this,  but 
there  are  very  few  who  can  give  a  reason  why  the  iron  is 
so  strong. 

Just  think  of  the  rod  being  made  up  of  molecules,  as 
we  have  learned  that  all  bodies  are.  These  molecules 
would  fall  apart  if  there  were  not  something  to  hold 
them  together.  They  are  held  together  by  attraction, 
and  the  iron  is  strong  just  because  this  attraction  is  very 
strong. 

How  are  the  molecules  of  any  body  held  to- 
gether ? — Just  as  there  is  attraction  among  the  molecules 
of  iron,  so  there  is  among  the  molecules  of  any  other  solid 
body  an  attraction  which  holds  them  together.  This  at- 
traction acts  continually.  Were  it  to  stop  its  action  for 
the  briefest  moment,  solid  bodies  would  be  seen  instantly 
crumbling  to  pieces.  Chairs,  stoves,  tables,  and  indeed 
the  very  walls  of  the  house,  would  fall  to  powder  finer 
and  looser  than  ashes  or  flour. 

The  attraction  which  holds  the  molecules  of  a  body 
together  is  called  cohesion^ 

Is  the  cohesion  alike  in  all  bodies  ? — Cohesion  is 
much  stronger  in  some  bodies  than  in  others.  Iron  is 
very  cohesive  but  lead  is  not.  It  is  easy  to  break  a  small 
rod  of  lead,  while  a  rod  of  iron,  of  the  same  size,  would 
resist  all  our  power.  It  is  because  the  cohesion  is  so 
strong  in  iron  that  this  metal  is  so  well  adapted  to  use  in 
making  carriages,  in  building  bridges,  and  in  many  other 
arts  which  you  can  easily  mention,  where  great  strength 
is  needed. 

If  cohesion  is  strong  enough  to  bind  the  molecules  of  a 
body  firmly  together,  the  body  is  a  solid  ;  but  if  it  is  very 
feeble  indeed,  the  body  is  a  liquid. 


22  NATURAL  PHILOSOPHY. 

Are  particles  of  different  kinds  of  matter  held 
together? — There  is  also  an  attraction  between  particles 
of  different  kinds  of  matter.  When,  for  example,  one 
writes  upon  the  blackboard,  he  leaves  fine  particles  of  the 
crayon  clinging  to  the  surface  of  the  board.  Particles  of 
water  cling  to  the  hand  that  is  withdrawn  from  a  bath  in 
water;  and  it  may  be  that  particles  of  soil,  clinging  to  the 
hand  unpleasantly,  made  the  bath  necessary  in  the  first 
place. 

In  all  such  cases  we  notice  that  there  is  an  attraction 
between  particles  of  different  kinds  of  matter.  Attraction 
between  particles  of  unlike  kinds  is  called  adhesion. 

By  what  experiment  may  we  illustrate  it  ?— A 
very  prettv  experiment  is  shown  in  Fig.  2.     It  illustrates 
Fi?-2-  the  adhesion  between  water  and 

brass.  A  round  plate  of  brass, 
having  a  handle  fastened  to  its 
center,  is  laid  flat  upon  the  sur- 
face of  water,  and  then  slowly 
and  gently  lifted.  The  water  un- 
der it  is  also  lifted  a  little,  as  the 
picture  shows  it. 

You  can  use  a  plate  of  wood  or 
of  glass  in  the  same  way. 
In  what  curious  way  may  the  adhesion  between 
solids  and  liquids  be  shown  ? — If  you  will  take  two 
pieces  of  glass  and  put  them  side  by  side  no  farther  apart 
than  the  thickness  of  a  sheet  of  paper,  and  will  then  bring 
their  lower  edges  carefully  in  contact  with  the  surface  of  some 
colored  water,  you  will  see  that  fluid  suddenly  spring  up 
an  inch  or  two  between  the  plates  arid  remain  standing  at 
that  height.  In  fact,  it  will  stay  up  between  the  plates  even 
if  you  lift  them  quite  away  from  the  water.  It  must  be 


ATTRACTION.  23 

the  attraction  between  the  water  and  the  glass  which  lifts 
the  fluid  and  holds  it  up  between  the  plates. 

What  is  the  effect  when  the  plates  are  not  paral- 
lel ? — Still  more  curious  is  the  effect  if  you  will  put  the' 
plates  so  that  their  edges  will  be  nearer  together  at  one 
side  than  at  the  other.  The  water  jumps  up  as  before, 
but  its  upper  edge,  instead  of  being  horizontal  as  it  was 
in  the  other  experiment,  will  be  in  the  form  of  a  beautiful 
curve.  The  liquid  rises  highest  where  the  plates  are  near- 
est_togethe£-. 

Suppose  small  tubes  be  used  instead  of  plates. — 
When  small  glass  tubes  are  used  instead  of  plates,  the  fluid 
will  rise  still  higher — just  twice  as  high  as  between  plates 
whose  distance  apart  is  equal  to  the  diameter  of  the  tube 
used. 

It  has  also  been  proved  that  the  liquid  will  rise  highest 
in  the  smallest  tube.  It  will  rise  two  times  as  high  in  a 
tube  whose  diameter  is  only  one  Jtalf  as  great  as  that  of 
another. 

What  is  the  law?— The  law  is  this:  the  height  to 
which  the  fluid  rises  is  inversely  as  the  "diameters  of 
the  different  tubes."  If,  for  example,  one  tube  is  three 
times  the  diameter  of  another,  water  will  rise  in  it  only 
one  third  as  high. 

What  other  examples  of  this  action  ?— Water  soaks 
upward  through  parous  soils,  and  by  this  means  they  are 
kept  moist  and  fertile.  The  oil  is  lifted  through  the  lamp- 
wick  to  supply  the  flame  above.  Ink  spreads  through 
blotting-paper  when  only  one  corner  of  it  touches  the  drop. 
All  these  and  many  other  familiar  tilings  that  might  be 
named,  are  caused  by  the  same  influence  which  lifts  water 
in  small  tubes  or  between  glass  plates.  It  is  an  attraction 
between  solid  and  liquid  bodies. 


2-J-  NATURAL  PHILOSOPHY. 

This  attraction  between  solid  and  liquid  bodies  is  very 
generally  called  capillary  attraction,  but  it  is  really  nothing 
different  from  adhesion. 

Give  examples  of  the  action  of  gravitation. — A 

stone  dropped  from  the  hand  falls  swiftly  to  the  ground, 
because  there  is  an  attraction  between  the  earth  and  the 
stone.  An  apple  bends  its  stem  because  the  same  kind  of 
attraction  is  pulling  it  toward  the  earth,  and  when  the 
fruit  ripens  and  the  stem  has  grown  weaker,  the  same 
force  causes  the  apple  to  break  away  and  fall.  This  at- 
traction is  called  gravitation.  It  is  the  attraction  which 
acts  upon  all  bodies  and  through  all  distances. 

Give  examples  of  pressure  caused  by  gravita- 
tion.— Gravitation  not  only  causes  a  body  to  fall  if  left 
without  support,  it  also  causes  one  body  to  press  upon 
another  on  which  it  rests.  A  stone  press  :s  heavily  upon 
the  ground  because  gravitation  is  pulling  it  downward. 
All  things  upon  the  earth  are  held  there,  and  exert  their 
pressure,  because  gravitation  is  acting  upon  them.  Some 
are  held  with  much  more  force  than  others,  as  we  may 
easily  learn  by  trying  to  lift  them.  A  pail  of  water  hangs 
heavily  upon  the  arm  because  gravitation  is  pulling  it 
down. 

What  is  weight  ? — It  is  easier  to  lift  a  block  of  wood 
than  a  stone  of  the  same  size,  because  gravitation  is  pulling 
the  stone  down  with  more  power.  To  say  that  the  stone 
is  heavier  than  the  wood  means  just  the  same  as  to  say 
that  the  attraction  of  the  earth  upon  the  stone  is  stronger 
than  upon  the  wood.  Indeed,  the  wcV^jofji^Jwi^is 
Qnljjhe  measure  of  the  attraction  whjclTthe  earth  exerts 
upon  it. 

How  do  we  tell  whether  two  bodies  have  equal 


ATTRACTION.  25 

weights  ?— A  pair  of  scales  enable  us  to  tell  whether 
bodies  have  equal  weights.  If  we  put  one  body  in  each 
scale-pan  and  the  two  are  balanced,  we  know  that  gravita- 
tion is  pulling  one  down  just  as  much  as  the  other;  in 
other  words,  the  two  bodies  are  equal  in  weight. 

What  are  sets  of  weights  ? — A  piece  of  metal  upon 
which  the  earth  exerts  a  certain  amount  of  attraction  may 
be  called  an  ounce  weight  j  then  another  upon-  which  the 
attraction  is  twice  as  great  is  called  a  two-ounce  weight ; 
and  if  upon  a  third  the  attraction  is  sixteen  times  as  great 
as  upon  the  first,  it  would  be  a  sixteen-ounce  weight,  or  a 
pound  avoirdupois.  Several  such  pieces  of  metal,  made 
with  care  to  represent  the  various  units  of  weight,  form 
what  is  called  a  "  set  of  weights,"  to  be  used  in  weighing 
the  various  articles  in  trade. 

In  what  direction  does  the  earth  attract 
bodies  ?  —  The  earth  attracts  all  bodies  toward  its 
center.  From  whatever  point  a  ball  is  dropped,  it  will 
fall  in  a  straight  line  toward  the  center  of  the  earth. 

This  direction  is  always  perpendicular  to  the  surface 
of  still  water.  You  can  easily  examine  this  fact  yourself 
by  fastening  a  string  to  some  heavy  body  and  then  hold- 
ing or  hanging  it  over  a  vessel  of  water,  as  you  see  it  in 
the  picture.  (Fig.  3,  p.  26.)  The  string  shows  the  direc- 
tion of  the  force  of  gravity  *  exactly,  and  it  is  easy  to  see 
that  it  is  perpendicular  to  the  surface  of  the  water. 

Such  a  cord  and  ball  is  called  a  plumb-line :  builders 
use  it  to  find  out  whether  their  walls  are  vertical. 

Does  gravity  always  cause  motion  downward  ? 
— While  the  earth's  attraction  is  forever  downward,  yet 
it  does  sometimes  produce  motion  upward.  For  ex- 

*  The  earth's  attraction  is  sometimes  called  gravity. 


NATURAL  PHILOSOPHY. 
Fig.  8. 


ample,  it  lifts  the  higher  pcale-pan  of  a  balance  by  pull- 
ing the  other  downward  at  the  same  time,  with  greater 
force. 

In  the  same  way  gravitation  causes  the  upward  motion 
of  smoke  by  pulling  the  heavier  air  down  under  it,  thus 
pushing  it  upward. 

Why  does  a  cork  rise  in  water?— One  more  illus- 
tration will  be  enough.  A  cork  at  the  bottom  of  a  vessel 
of  water  quickly  rises  to  the  surface.  Now  it  does  not  rise 
because  it  is  light,  as  many  people  will  say  it  does.  The 


ATTRACTION.  27 

fact  is  that  gravitation  pulls  both  the  cork  and  the  water 
downward,  but  it  pulls  the  water  with  the  greatest  force. 
The  water  must  go  down  under  the  cork,  and  in  doing  so 
must  push  the  cork  upward. 

Is  the  earth  attracted  by  small  bodies  ? — To  say 
that  the  earth  attracts  an  apple  is  not  more  true  than  to 
say  that  the  apple  attracts  the  earth.  The  truth  is  simply 
that  they  attract  each  other.  The  earth  attracts  every 
body  great  and  small,  and  every  one  attracts  the  earth  in 
return.  Every  leaf  and  every  rain-drop  or  snow-flake  that 
falls  to  the  ground  attracts  the  earth  just  as  truly  as  it  is 
attracted  by  it. 

Is  the  earth  moved  by  this  attraction  ?— The  earth 
attracting  the  rain-drop,  makes  it  fall  toward  the  ground  ; 
the  rain-drop  attracts  the  earth  in  return  :  can  we  suppose 
that  the  great  earth  moves  up  to  meet  it  ?  We  have  seen 
thousands  of  rain-drops  fall,  but  who  ever  saw  the  earth 
go  up  to  meet  them  !  And  yet  perhaps  it  does,  for  we 
could  not  see  it  if  it  did.  The  attraction  would  make 
the  drops  go  as  many  times  farther  than  it  would  the 
earth,  as  the  earth  is  times  heavier  than  the  drops,  and  it 
would  not  be  possible  to  see  the  motion  through  so  small 
a  distance. 

Is  gravitation  confined  to  the  earth? — But  this 
force  is  not  confined  to  the  earth  and  the  bodies  near  its 
surface :  the  sun  and  all  the  other  bodies  in  the  heavens 
attract  each  other.  It  is  exerted  by  every  body  of  matter 
upon  every  other  in  the  universe.  Grains  of  sand  are 
held  by  it  in  their  places  on  the  sea-shore,  and  it  keeps 
the  sea  itself  from  rising  out  of  its  bed.  It  is  at  the  same 
time  acting  upon  the  earth  itself  and  upon  all  the  other 
heavenly  bodies  to  keep  them  in  their  orbits. 

On  -what  does  the  strength  of   this  force  de- 


28  NATURAL  PHILOSOPHY. 

pend? — The  strength  of  gravitation  depends  upon  two 
things  :  first,  upon  the  quantity  of  matter  in  the  body 
exerting  it ;  and  second,  upon  the  distance  through  which 
it  acts. 

If  the  quantity  of  matter  is  doubled  the  attraction  will 
be  doubled  also.  Or,  in  general  terms,  the  attraction  is 
in  proportion  to  the  quantity  of  matter  exertiny  it. 

But  it'  the  distance  be  doubled  the  attraction  will  be 
only  one  fourth  as  strong.  At  three  times  the  distance 
the  force  is  only  one  ninth  as  strong.  In  general  terms 
we  say,  the  attraction  is  inversely  as  the  square  of  the  dis- 
tance between  the  bodies. 

What  is  the  center  of  gravity  of  a  body  ?— Boys 
are  sometimes  very  fond  of  balancing  books  or  ball- 
clubs,  or  even  long  poles,  upon  the  end  of  a  finger.  They 
often  become  very  skillful  in  doing  this,  without  knowing 
that  every  time  they  do  it  they  are  trying  an  experiment 
in  natural  philosophy.  The  fact  which  the  experiment 
illustrates  is  this  :  there  is  a  point  in  every  body  wfcich  if 
supported  the  whole  body  will  be  at  rest.  This  point  is 
called  the  center  of  gravity.  The  ball-club  has  a  center 
of  gravity,  and  if  the  finger  can  be  kept  exactly  under  that 
point  the  club  will  not  fall. 

Illustrate  by  using  a  ruler. — Or,  to  study  this  sub- 
ject further,  let  a  ruler  be  balanced  across  your  finger. 
There  will  be  just  as  much  of  the  weight  of  the  ruler  on 
one  side  of  the  finger  as  on  the  other,  and  a  point  exactly 
over  the  finger  and  in  the  middle  of  the  ruler  is  the  center 
of  the  weight  of  the  ruler,  or,  as  we  have  already  named  it, 
the  center  of  gravity. 

Every  body  has  a  center  of  gravity,  and  when  this  point 
is  supported  the  whole  body  will  be  at  rest. 

Where  is  the  center  of  gravity  of  a  body? — The 


ATTRACTION.  29 

center  of  gravity  is  not  always  in  the  center  of  the  body. 
Suppose  one  end  of  your  ruler  to  be  loaded  with  lead  :  you 
would  then  have  to  put  your  finger  nearer  to  the  heavier 
end  in  order  to  balance  it ;  the  center  of  gravity  would 
be  nearer  to  the  loaded  end. 

When  two  boys  are  playing  at  seesaw  the  support  of 
the  board  must  be  under  the  center  of  gravity,  but  if  the 
boys  are  not  of  the  same  weight  the  support,  as  every  one 
knows,  must  be  nearer  to  the  heaviest  boy.  The  center  of 
gravity  of  the  board  and  boys  together  is  nearer  to  the 
end  where  the  large  boy  sits. 

In  Fig.  4  the  center  of  gravity  in  each  body  is  at  G. 

Fig.  4 


"What  is  the  line  of  direction  ?— Now  imagine  a 
vertical  line  drawn  through  the  center  of  gravity,  as  shown 
by  the  vertical  dotted  lines  in  Fig.  4.  This  line  ^  will 
show  the  direction  in  which  the  body  would  fall  if  it 
were  left  without  support,  and  it  is  called  the  line  of  direc- 
tion. 

Where  may  we  place  the  support  for  the  center 
of  gravity? — We  may  support  the  center  of  gravity  by 
placing  the  support  at  any  point  in  the  line  of  direction. 
It  may  be  placed  at  the  center  of  gravity,  or  at  some 


30  NATURAL  PHILOSOPHY. 

point  above  it,  or  at  some  point  below  it.     Fig.  5  shows 
a  disk  of  metal  supported  in  these  three  ways. 


Fig.  5. 


Describe  three  kinds  of  equilibrium.— When  all 

parts  of  a  body  are  balanced  it  is  said  to  be  in  equilibrium. 
Now  when  the  support  is  at  the  center  of  gravity,  as 
shown  in  the  middle  disk  of  the  figure,  the  body  is  said 
to  be  in  indifferent  equilibrium,  because  it  will  rest  as 
well  in  one  position  as  another. 

When  the  support  is  placed  above  the  center  of  gravity, 
as  in  the  disk  at  the  right  hand,  the  body  is  said  to  be  in 
stable  equilibrium,  because  it  will  not  rest  as  well  in  any 
other  position. 

When  the  support  is  placed  exactly  below  the  center  of 
gravity,  as  in  the  disk  at  the  left  hand,  the  body  is  said  to 
t>e  in  unstable  equilibrium,  because  the  slightest  force  will 
push  it  over. 

That  a  body  may  stand,  where  must  the  line  of 
direction  pass? — If  the  line  of  direction  passes  through 
any  point  in  the  base  on  which  the  body  is  placed,  the 
body  will  stand,  but  if  this  line  passes  outside  of  the  base, 


ATTRACTION.  3  [ 

the  body  must  fall.  The  leaning  cylinder  in  Fig.  4r  does 
not  fall,  because  the  line  of  direction  passes  through  the 
base,  and  hence  the  center  of  gravity  is  supported  ;  but  if 
it  should  lean  a  little  more  this  line  would  pass  outside 
the  base,  and  the  cylinder  would  tip  over.  A  table  stands 
very  firm  because  it  is  not  easy  to  tip  it  so  far  that  the 
line  of  direction  would  pass  outside  the  base. 

Carriages  may  lean  considerably  to  one  side  without 
overturning  (Fig.  6),  but  an  accident  is  sure  to  happen  if 

Fig.  & 


/hey  lean  so  far  as  to  throw  the  line  of  direction  beyond 
the  lower  side  of  the  wheel. 

Upon  what  does  the  stability  of  a  body  de- 
pend ? — Now  some  bodies  stand  more  firmly  than  others, 
and  in  looking  for  the  reason  we  find  that  the  stability 
of  a  body  depends  upon  two  things.  The  first  of  these  is, 
the  height  of  the  center  of  gravity  above  the  base. 

A  wagon  loaded  with  hay  overturns  easily,  while  if 
loaded  with  stone  it  would  pass  the  same  spot  in  the  road 
with  perfect  safety.  The  center  of  gravity  of  the  load  of 
hay  is  so  much  higher,  that  to  lean  a  little  throws  the  line 


32  NATURAL  PHILOSOPHY. 

of  direction  beyond  the  wheel.  The  higher  the  center  of 
gravity  of  any  body  is,  the  more  unstable  will  it  be. 

What  else  influences  the  stability  of  a  body  ? — 
The  size  of  the  base  is  the  second  thing  that  influences 
the  stability  of  a  body.  It  will  of  course  be  more  difficult 
to  tip  the  line  of  direction  beyond  a  large  base  than  be- 
yond a  small  one.  A  narrow  boat  overturns  more  easily 
than  a  wide  one,  or,  to  mention  an  example  which  you 
may  see  at  any  time,  a  thick  book  will  stand  upon  its  end 
more  firmly  than  a  thin  one  of  the  same  height. 

We  see,  then,  that  the  lower  the  center  of  gravity  and 
the  broader  the  base,  the  firmer  will  the  body  stand. 

Mention  illustrations  of  these  principles. — Illus- 
trations of  tliese  principles  of  center  of  gravity  are  among 
the  most  common  affairs  of  life.  Indeed,  we  unconsciously 
apply  them  in  almost  every  motion  and  position  of  our 
own  bodies. 

When  standing,  the  base  upon  which  the  body  rests  is 
the  space  between  the  feet ;  the  center  of  gravity  must  be 

Fig.  7- 


^=LV  — = 


kept  over  this  base  or  the  person  will  fall.     In  carrying  a 
pail  of  water  we  unconsciously  lean  to  the  other  side,  and 


ATTRACTION.  33 

if  the  load  is  very  heavy  we  at  the  same  time  stretch  ont 
the  opposite  arm  (Fig.  7).  The  pack-peddler  leans  for- 
ward, for  it  he  did  not  the  heavy  load  would  throw  the 
center  of  gravity  behind  his  feet  and  he  would  tumble 
backward. 

What  illustration  does  the  showman  furnish  ? — 
The  showman  oifers  a  gold  coin  to  the  boy  who  will  stand 
with  his  heels  pressed  against  the  wall  of  a  room  and  then, 
pick  it  from  the  floor  in  front  of  him  without  falling.  He 
is  perfectly  safe  in  making  the  offer.  For  no  one  can 
stoop  without  falling,  unless  when  he  throws  his  head 
forward  he  can,  at  the  same  time,  throw  some  other  part 
of  his  body  backward  far  enough  to  keep  his  center  of 
gravity  over  his  feet.  He  can  not  do  this  with  his  heels 
pressed  against  a  wall. 

Why  is  a  child  so  long  learning  to  walk? — When 
we  think  how  narrow  the  base  is  on  which  a  child  must 
stand,  being  just  the  space  on  the  floor  between  its  little 
feet,  and  then  how  high  is  the  center  of  gravity  of  his 
body,  we  need  not  wonder  that  he  is  so  long  a  time  in 
learning  to  walk.  The  many  falls  and  bruises  which  the 
little  one  gets  mark  his  failures  in  the  art  of  supporting 
the  center  of  gravity  always  over  the  base. 

How  may  we  try  the  experiment  ? — Let  one  who 
has  forgotten  how  hard  it  was  for  him  to  learn  to  walk 
refresh  his  memory  by  trying  to  walk  on  stilts.  Skill  in 
this,  like  that  of  the  child  in  walking,  needs  only  the 
power  to  keep  the  center  of  gravity  of  the  body,  every 
moment,  over  some  point  in  the  base. 


LIQUIDS. 


How  do  liquids  differ  from  solids  ? — The  molecules 
of  every  solid  substance  are  held  together  so  that  the 
body  will  keep  whatever  form  you  may  choose  to  give  it; 
but  in  water  the  molecules  are  held  together  with  such 
feeble  force  that  they  can  move  among  themselves  with 
the  greatest  ea"se,  and  you  can  not  give  it  any  shape  but 
that  of  the  vessel  which  holds  it. 

Water  and  other  substances,  in  which  cohesion  is  so 
slight  that  the  molecules  move  freely  among  themselves, 
are  called  liquids. 

Is  there  any  cohesion  in  liquids  ?— Still  the  cohe- 
sion in  a  liquid  is  strong  enough  to  be  detected.  Look 

again  at  Fig.  2,  and  notice 
that  the  water  would  not  be 
lifted  under  the  disk,  as  it 
is  there  shown  to  be,  unless 
the  particles  of  water  cling 
to  each  other.  This  shows 
cohesion  among  them.  The 
drop  of  dew  collected  upon 
a  leaf  (Fig.  8)  shows  cohe- 
sion in  water,  for  what 
else  could  hold  the  parts  of 
the  drop  together  ? 

How  can  we  judge  of  its  strength  ? — To  get  a  bet- 


LIQUIDS.  35 

ter  idea  of  the  force  of  cohesion  in  water,  we  may  watch 
it  dripping  from  some  support.  A  drop  grows  larger 
while  clinging  to  its  support,  until  at  last  it  breaks  away. 
The  weight  of  the  drop  just  at  the  moment  when  it  breaks 
away  is  just  enough  to  pull  the  molecules  of  water  apart, 
and  measure*  the  cohesion  in  the  liquid.  The  liquid  in 
which  there  is  the  greatest  cohesion  will  give  the  largest 
drops. 

Is  -water  compressible  ? — A  famous  experiment  was 
made  at  Florence  about  a  hundred  years  ago  to  find  out 
whether  water  could  be  compressed.  A  hollow  globe  of 
gold  was  tilled  with  water,  and  then  the  opening  sealed  so 
very  tight  that  no  water  could  pass  it.  An  enormous 
pressure  was  then  put  upon  the  globe,  when,  to  the  sur- 
prise of  all,  the  water  oozed  through  the  pores  of  the 
metal.  This  experiment  seemed  to  prove  that  water  was 
not  compressible.  But  more  careful  experiments  have 
since  shown  that  water  is  compressible.  It  is  in  so 
slight  a  degree  that  the  Florentine  experiment  was  too 
rude  to  show  it  at  all.  It  needs  a  pressure  of  15  Ibs. 
upon  every  square  inch  of  the  surface  of  the  vessel  in 
which  the  water  is  held  to  compress  the  fluid  .0000503  of 
its  bulk. 

Is  water  elastic  ?— The  instant  that  the  pressure  is 
removed  from  the  compressed  water  it  springs  back  to  its 
former  bulk,  and  this  proves  it  to  be  elastic. 

What  is  more  remarkable,  it  springs  back  with  exactly 
as  much  force  as  was  exerted  to  compress  it.  *  When  com- 
pressed only  .0000503  of  its  bulk  it  will  spring  back  with 
a  force  of  15  Ibs.  to  a  square  ine\  Now  when  a  body 
springing  back  restores  all  the  force  that  compressed  it, 


NATURAL  PHILOSOPHY. 


Fig.  9. 


it  is  paid  to  be  perfectly  elastic.  Water  and  other  liquids 
are  perfectly  elastic. 

What  shows  the  downward  pressure  of  water  ? 

— The  downward  pressure  of  water  is  shown  by  its  wreight. 
To  lift  a  pailful  of  water  you  must  overcome  its  down- 
ward pressure.  This  may  tax  your  strength  severely, 
because,  if  the  pail  holds  one  cubic  foot  of  the  liquid,  you 
must  lift  a  weight  of  G2£  Ibs.  ;  a  cubic  foot  of  water 
weighs  62|  Ibs. 

Does  water  exert  pressure  upward? — To  learn 
whether  water  presses  upward  as  well  as  downward  the 
following  experiment  (Fig.  9)  may  be  tried.  A  plate  of 

metal  is  hung  from  the 
end  of  a  string,  which  is 
passed  through  a  glass 
tube  open  at  both  ends. 
By  means  of  this  string 
the  plate  of  metal  may  be 
held  tightly  against  the 
lower  end  of  the  tube. 
Now  if  this  end  of  the 
tube  is  pushed  down  into 
a  vessel  of  water,  the  string 
may  be  dropped  and  the 
plate  of  metal  will  still 
stay  up  against  the  glass. 
By  a  moment's  thought 
you  see  that  it  must  be 
the  water  that  holds  the 
heavy  metal  up,  and  that  to  do  this  it  must  exert  an 
upward  pressure. 
Does  water  exert  pressure  sidewise  ? — The  same 


LIQUIDS. 


37 


experiment  shows  that  water  exerts  a  pressure  sidewise, 
for  you  may  find  that  the  water  is  gradually  pushed  side- 
wise  between  the  plate  and  the  end  of  the  tube,  slowly 
filling  the  tube  with  water. 

In  -what  direction  does  -water  exert  pressure  ? — 
In  fact,  water  and  other  liquids,  when  at  rest,  exert  press- 
ure in  all  directions.  And  another  fact  should  be  remem- 
bered ;  it  is,  that  the  pressure  at  any  point  is  equal  in  all 
directions.  If,  for  example,  there  is  at  any  point  a  down- 
ward pressure  of  10  Ibs.  there  will  be,  at  the  same  time, 
a  pressure  of  10  Ibs.  upward  and  sidewise,  and  indeed  in. 
every  possible  direction. 

The  pressure  of  water  in  several  Fi?- 10- 

directions    at   once   is   very   well 
shown  in  Fig.  10. 

Why  is  the  surface  of  wa- 
ter at  rest  always  level  ? — It 
is  because  water  presses  equally 
in  all  directions  that  a  body  of 
water  can  not  be  quiet  unless  its 
upper  surface  is  level. 
•  Let  us  explain  this  more  fully. 
The  molecules  of  water  move  so 
easily  that  if  the  pressure  in  one 
direction  is  never  so  little  more 
than  in  another,  the  liquid  will 

move.  Now  if  the  downward  pressures  at  all  points  are 
equal,  all  the  other  pressures  must  be  equal  too,  and  the 
water  will  not  move,  but  the  downward  pressures  will  not 
be  equal  at  all  points  unless  the  surface  of  the  water  is 
level,  and  for  this  reason  the  water  can  not  rest  unless  its 
surface  is  level. 

The  wind  may  cover  the  surface  of  the  sea  with  ripples 


233560 


38  NATURAL  PHILOSOPHY. 

or  lash  it  into  billows ;  but  let  the  wind  be  hushed,  and  the 
ripples  or  billows  will  gradually  sink  into  a  surface 
smoother  than  that  of  the  most  polished  mirror,  just 
because  the  pressure  in  all  directions  can  not  be  made 
equal  without. 

Will  the  shape  of  the  vessel  make  any  differ- 
ence?— No  matter  how  irregular  the  form  of  a  vessel 
may  be,  all  parts  of  the  surface  of  the  water  in  it  roust  be 
at  the  same  height,  or  in  other  words  level.  The  vessel 
shown  in  Fig.  11  has  a  very  irregular  shape.  There  is 

Fig.  11. 


first  the  large  vase  at  the  left  hand,  then  the  horizontal 
tube,  and  finally  the  tubes  reaching  upward  from  the 
last;  yet  it  is  all  one  vessel,  because  the  water  can  pass 
freely  from  one  part  to  another. 

If  water  is  poured  into  the  vase  it  will  rise  just  as  fast 
in  the  tubes,  and  will  at  last  stand  at  the  same  height  in 
all  parts,  as  the  picture  shows. 

How  are  cities  supplied  with  water?— It  is  on 


LIQUIDS.  39 

this  principle  that  many  cities  are  supplied  with  water. 
Water  from  the  streams  of  the  country  around  is  led  into 
a  reservoir  where  its  surface  will  be  higher  than  the  city. 
A  large  pipe  is  then  laid  under-ground,  reaching  from  the 
reservoir  down  to  the  city,  and  branches  from  it  are  laid 
under  the  streets.  From  these  main  pipes -a  branch  goes 
into  each  house  which  is  to  receive  the  water,  and 
reaches  up  to  the  room  where  the  water  is  to  be  drawn. 
Now  the  water  will  rise  in  these  pipes  as  high  as  the 
surface  of  that  in  the  reservoir,  if  they  will  allow  it  to  do 
so,  and,  of  course,  if  one  be  opened  anywhere  below  that 
level  the  water  will  flow  from  it. 

How  are  fountains  produced  ?— If  the  pipe  which 
is  bringing  water  from  a  reservoir  does  not  rise  as  high 
as  the  reservoir,  the  water  will  spout  upward  in  the  form 
of  a  fountain.  In  Fig.  11  one  of  the  tubes  is  shorter 
than  the  others,  but  the  water  rises  almost  as  high  as  it 
does  in  them  :  being  thrown  into  the  air  instead  of  rising 
in  a  pipe,  we  call  it  a  fountain. 

Upon  what  does  the  pressure  of  -water  on  the 
bottom  of  the  vessel  which  holds  it  depend?— 
Suppose  the  bottoms  of  two  vessels  are  the  same  in  size, 
but  that  one  vessel  is  twice  as  high  as  the  other.  When 
both  are  filled  with  water  it  is  found  that  there  will  be 
just  twice  as  much  pressure  on  the  bottom  of  the  highest. 
If  one  is  three  times  as  high  as  the  other,  the  pressure  on 
its  bottom  will  be  three  times  as  great. 

The  pressure  upon  the  bottom  of  a  vessel  *  of  water  is 
always  just  in  proportion  to  the  height  of  the  water. 

Does  not  the  shape  of  the  vessel  make  a  differ- 
ence ? — We  may  take  vessels  of  very  different  shapes,  but 
if  they  are  filled  with  water  to  the  same  height,  arid  if  their 
bottoms  are  of  the  same  size,  the  pressure  on  the  bottom 


40  NATURAL  PHILOSOPHY. 

will  be  the  same  in  all.  Suppose,  for  example,  that  each 
vessel  has  a  bottom  whose  surface  is  ten  square  inches : 
one  of  them  may  be  just  as  large  at  the  top  as  at  the 
bottom,  another  may  be  larger  at  the  top,  and  another 
smaller ;  but  when  they  are  filled  to  the  same  height  with 
water  the  pressure  upon  the  bottoms  will  be  alike  in 
all. 

The  pressure  upon  a  bottom    of  given    size    depends 
I  ,  entirely  upon  the  height  of  the  water  above  it. 
•%£       How  may  a  little  water  exert  very  great  press- 
ure ? — We  may  now  notice  a  curious  fact,  which  seems 
at  first  to  be  impossible.     A  very  small  quantity  of  water 
may  exert  an  enormous  pressure. 

Fig.  12  shows  how  this  may  be  proved.  In  the  first 
place,  a  very  tight  cask  is  filled  with  water  and  a  tall  tube 
is  afterward  screwed  into  the  top.  By  filling  this  tube 
with  water  the  cask,  unless  uncommonly  strong,  will  be 
broken  asunder.  The  very  small  quantity  of  water  in 
the  tube,  no  more  than  a  child  could  lift,  exerts  a  pressure 
strong  enough  to  break  the  staves  of  the  cask. 

How  can  this  be  explained  ? — Suppose  the  end  of 
the  tube  is  -^  of  a  square  inch,  and  that  the  tube  is  high 
enough  to  hold  a  pound  of  water.  The  pressure  on  -gL-  of 
an  inch  would  be  one  pound,  and  on  a  whole  inch  it 
wrould  be  50  Ibs.  And  since  water  presses  equally  in  all 
directions  there  would  be  50  Ibs.  pressure  on  every  square 
inch  of  the  inside  surface  of  the  cask.  Such  a  pressure  is 
more  than  the  cask  can  bear. 

Would  any  other  equal  pressure  have  the  same 
effect? — Any  other  pressure  equal  to  the  weight  of  the 
column  of  water  in  the  tube  would  have  the  same  effect. 
The  pressure  of  your  hand,  or  of  a  pound-weight  of  metal, 
might  take  the  place  of  the  pound  of  water  in  the  tube ; 


41 


42  NATURAL  PHILOSOPHY. 

the  pressure,  exerted  in  any  way,  would  be  transmitted 
equally  in  all  directions  and  break  the  cask. 

Show  how  a  light  weight  may  balance  a  heavy 
one. — Now  suppose  two  cylinders,  one  just  twice  as  large 
as  the  other,  to  be  joined  together  by  a  tube  at  their  bot- 
toms (Fig.  13),  and  let  there  be  a  piston  fitting  each  cylin- 
Fig.  is-  der  exactly,  and  carrying  a  table 

as  the  picture  shows  them.  Now 
if  a  one-pound  weight  be  put  upon 
the  small  table  it  will  balance  a 
two-pound  weight  upon  the  other. 
If  one  cylinder  were  one  hun- 
dred times  larger  than  the  other, 
then  one  pound  on  the  small  table 
would  balance  one  hundred  pounds 
on  the  large  one. 
What  machine  acts  on  this  principle  ? — The  hy- 
drostatic press  is  made  to  act  on  this  principle.  The 
piston  in  the  small  cylinder  is  pushed  down  by  hand,  or 
perhaps  by  a  steam-engine,  while  any  thing  to  be  pressed 
is  put  between  the  large  table  and  a  solid  pressure-plate 
built  above  it. 

This  machine  is  used  for  pressing  hay  and  cotton  into 
bales,  for  testing  the  strength  of  ropes,  and,  in  a  word,  it 
is  preferred  to  any  other  machine  whenever  a  great  press- 
ure is  to  be  exerted. 


GASES. 


How  do  gases  differ  from  liquids  ? — In  water  and 
in  other  liquids  there  is  a  slight  degree  of  cohesion,  but  in 
air  and  other  gjises  there  is  no  cohesion  at  all.  The  mole- 
cules of  air  are  trying  to  get  just  as  far  away  from  each 
other  as  possible  at  all  times  ;  and  this  is  true  also  of  all 
bodies  in  the  form  of  air,  or,  as  they  are  called,  gases. 

Air  is  the  most  common  of  all  gases,  and  on  this  account 
it  is  used  to  illustrate  the  properties  of  this  class  of  bodies. 

Is  the  air  expansible  ? — An  easy  and  pretty  experi- 
ment will  teach  us  whether  air  can  be  expanded.  Take  a 
small  vial  having  in  it  a  little  colored  water,  and  fasten 
into  its  neck  an  air-tight  cork,  through  which  a  small  tube 
just  reaches  into  the  bottle.  This  tube  should  be  several 
inches  long.  If  the  vial  be  held  bottom  upward  the  col- 
ored water  will  riot  run  into  the  tube,  but  if  the  lips  be 
applied  to  the  lower  end  of  the  tube,  and  the  air  be  drawn 
out,  the  colored  water  will  quickly  run  down.  This  shows 
that  the  air  above  the  water  in  the  vial  expands  to  push 
the  water  out. 

In  what  other  way  is  the  air  of  the  vial  ex- 
panded ?— If,  instead  of  taking  the  air  out  of  the  tube, 
you  gently  warm  the  vial,  you  will  see  the  colored  liquid 
move  out  of  the  vial  and  down  the  tube.  In  this  experi- 
ment the  air  is  expanded  by  heat. 


44:  NATURAL  PHILOSOPHY. 

Boys  sometimes  amuse  themselves  by  bursting  balloons 
or  bladders  tilled  with  air  by  warming  them.  They  thus 
illustrate  the  expansibility  of  air,  for  the  air  when  heated 
tries  to  till  more  room  than  it  did  when  cold,  and  in  trying 

f\s.  14.  to  get  larger  bursts  the  balloon  with  a  loud  re- 
port like  a  gun. 

Is  air  compressible  ?— If  we  take  a  cylinder 
with  a  piston  fitting  it  air-tight  (Fig.  14),  we  may 
easily  push  the  piston  down  some  distance  into 
the  cylinder.  No  air  gets  out,  but  the  piston, 
while  going  down,  crowds  the  air  along  before  it 
until  the  cylinder  may  be  less  than  half  full.  By 
greater  force  than  can  be  given  by  the  hand 
alone  the  piston  may  be  crowded  down  until  the 
cylinder  may  be  less  than  a  hundredth  or  a  thous- 
andth part  full. 

Is  air  elastic? — Compressed  air  will  spring 
back  to  its  original  bulk  when  the  pressure  is 
taken  away,  and  this  shows  that  it  is  elastic. 

It  is  also  found  that  air  after  being  compressed 
will  spring  back  with  just  as  much  force  as  was 
put  upon  it ;  this  shows  it  to  be  perfectly  elastic. 

Does  air  have  weight  ? — A  thin  globe  made 
of  glass  or  metal  is  weighed  when  full  of  air 
(Fig.  15).  The  air  is  then  taken  out  of  it  by 
means  of  an  air-pump,  soon  to  be  described,  and 
the  empty  globe  is  weighed.  The  globe  weighs  more 
when  full  of  air  than  when  empty,  and  this  proves  that 
air  has  weight. 

If  the  globe  will  hold  100  cubic  inches  of  air,  it  will 


CASES. 


4:5 


weigh  about  31  grains  less  when  empty,  and  this  shows 

that  100  cubic  inches  weigh  about  31  grains. 

Does  gravitation  act  upon  air  ? — The  weight  of  air, 

like  the  weight  of  wood  or  Fig.  is 

iron,  is  caused  by  the  at- 
traction     of      gravitation. 

Gravitation  acts   upon  the 

invisible    air    in    just    the 

same  way  that  it  does  upon 

water  or  upon  oil,  only  its 

action  is  not  so  strong. 
Try  this  experiment :  into 

a  tall  glass  jar  (Fig.  16)  or 

even  a  goblet  put  first  some 

water,  and    then    pour    in 

some  oil ;  the  oil  will  lie  on  top  of  the  water.     Afterward, 

if  you  can  have  some  mercury  you  will  be  able  to  pour  it 
Fig.  16.  into  the  jar  carefully  without 

disturbing  the  other  liquids. 
The  mercury  will  go  to  the 
bottom  and  form  a  layer  un- 
der the  others. 

Now  there  are  four  sub- 
stances in  the  jar,  arranged 
in  layers.  There  is  first  mer- 
curjr,  then  water,  then  oil,  and 
then  air.  And  they  are  in 
this  order  because  gravita- 
tion is  strongest  on  mercury, 
weaker  on  water,  weaker  yet 
on  oil,  and  weakest  on  air. 

For  a  similar  reason  the  water  of  the  sea  is  above  the 

rocks  and  then  the  atmosphere  above  the  water ;  but  if  grav- 


4t>  NATURAL  PHILOSOPHY. 

itation  did  not  act  upon  air  at  all,  the  atmosphere  would 
leave  the  earth  entirely  and  fly  off  into  space  beyond. 

In  -what  directions  does  air  exert  pressure  ? — The 
pressure  of  the  air  may  be  shown  in  a  very  simple  way. 
Cork  one  end  of  a  lamp-chimney,  and  stretch  a  piece  of 
caoutchouc  over  the  other.  Put  a  piece  of  pipe-stem 
tightly  through  the  cork,  and  the  apparatus  is  finished. 
.Now  with  the  lips  at  the  pipe-stem,  take  the  air  out  of  the 
chimney  and  you  will  see  the  caoutchouc  pushed  into  it. 
There  is  nothing  outside  to  push  it  into  the  tube  but  the 
air,  and  so  the  experiment  shows  the  pressure  of  the  air. 

Now  hold  the  tube  upward  or  downward,  or  in  any 
direction  whatever,  and  the  caoutchouc  will  be  pressed  in 
as  before ;  and  hence  we  see  that  the  air  presses  in  all 
directions. 

It  is  also  found  by  experiment  that  the  pressure  of  air 
in  all  directions  is  equal.  In  this  respect  air  and  water 
are  alike. 

The  finest  illustrations  of  the  pressure  of  the  air  may 
be  given  by  means  of  the  air-pump. 

Describe  the  air-pump. — In  this  instrument  there  is 
a  cylinder  (C,  Fig.  17),  with  a  tube  leading  from  the  bot- 
tom of  it.  The  other  end  of  this  tube  is  bent  upward  so 
as  to  pass  through  a  horizontal  plate  of  metal,  P.  At  the 
end  of  this  tube,  in  the  cylinder,  there  is  a  little  door  or 
valve,  as  it  is  called,  which  opens  upward,  and  will  open 
for  air  to  go  up,  but  will  shut  when  the  air  tries  to  go 
down  again.  In  the  cylinder  there  is  a  piston,  and  in  this 
there  is  another  valve  which  opens  upward.  The  plate, 
P,  is  so  smooth  that  a  glass  vessel,  R,  open  at  the  bottom, 
will  stand  upon  it  and  fit  so  closely  that  no  air  can  pass 
between  them.  This  vessel,  or  any  other  from  which  air 
is  to  be  taken,  is  called  a  receiver. 


GASES. 


Explain  the  action  of  the  pump. — When  the  piston 
is  lifted  the  air  in  the  receiver,  R,  will  expand,  and  a  part 


Fig.  IT. 


of  it  will  go  through  the  valve  at  the  bottom  of  the  cylin- 
der. When  the  piston  is  pushed  down  again,  the  air  in 
the  cylinder  will  push  its  way  through  the  valve  in  the 
piston.  When  the  piston  is  lifted  again,  the  air  above  it 
is  lifted  out  of  the  instrument  entirely,  while  another  part 
of  the  air  in  the  receiver  comes  through  the  valve  into  the 
cylinder.  And  in  this  way  every  upward  motion  of  the 
piston  pumps  a  part  of  the  air  out  from  the  receiver. 

The  air  can  be  so  nearly  pumped  out,  or  exhausted,  as 
it  is  usually  called,  that  there  will  not  be  enough  left  to 
lift  the  very  delicate  valves  of  the  instrument. 

How  -will  the  receiver  show  the  pressure  of  the 
air  ? — After  the  air  has  been  pumped  out  of  the  receiver, 
it  will  be  found  impossible  to  lift  it  away  from  the  pump- 
plate.  The  outside  air  presses  so  heavily  upon  it  that  if 
the  receiver  is  lifted  the  pump  will  rise  with  it. 


£8  NATURAL  PHILOSOPHY. 

How  may  the  pressure  be  shown  by  the  Magde- 
burg cups? — Fig.  IS  shows  the  Magdeburg  cups.     These 
Fig.  i&  cups  are  made  of  metal,  and  their  edges 

are  so  smooth  that  they  will  fit  each 
other  air  tight.  The  lower  one  may  be 
screwed  upon  the  pump-plate,  the  other 
then  placed  upon  it  and  the  air  taken 
out.  They  may  then  be  removed  from 
the  pump  without  letting  the  air  get  into 
them,  and  when  this  is  done,  the  pressure 
of  the  outside  air  will  hold  them  together 
with  great  force.  The  illustrious  Otto 
de  Guericke  of  Magdeburg,  who  invented 
the  air-pump,  and  to  whom  we  also  owe 
the  invention  of  these  cups,  made  a  pair  so 
large  that  it  needed  the  strength  of  four  horses  to  pull 
them  apart. 

How  may  the  pressure  be  shown  by  the  fountain 
in  vacuo  ? — The  pressure  of  air  is  shown  by  a  still  more 
beautiful  experiment,  represented  in  Fig.  19.  A  tall 
glass  receiver,  R,  made  air-tight,  has  a  tube  passing 
through  the  bottom.  The  lower  end  of  this  tube  may 
be  screwed  upon  the  plate  of  the  air-pump  :  the  other 
end  reaches  some  distance  into  the  vessel.  After  the  air 
is  exhausted  from  the  receivers,  if  the  lower  end  of  the 
tube  is  placed  in  water  and  the  valve  opened,  an  elegant 
fountain  will  be  thrown  up  inside  by  the  pressure  of  the 
air  upon  the  water  outside. 

How  may  the  upward  pressure  of  air  be  shown  ? 
— We  do  not  need  an  air-pump  to  show  the  upward 
pressure  of  air.  Just  take  a  common  bottle  and  fill  it  to 
the  brim  with  water ;  then  place  a  piece  of  paper  over  its 
mouth,  and  while  you  hold  the  paper  with  one  hand,  turn 


GASES.  49 

the  bottle  bottom  upward  with  the  other.     You  may  now 
let  go  of  the  paper  and  the  water  will  not  run  out  of  the 

Fig.  19. 


bottle.  The  water  and  the  paper  are  both  held  up  by  the 
upward  pressure  of  the  air.  If  the  neck  of  the  bottle  is 
very  small,  the  paper  need  not  be  used. 

How  may  the  downward  pressure  be  easily 
shown  ? — Take  a  tall  bottle  with  a  wide  mouth,  and  sink 
it  in  a  vessel  of  water,  and  when  full  of  the  liquid  lift  it 
gradually  with  its  bottom  upward  until  its  neck  only  is 
covered.  The  bottle  will  still  be  full  of  water.  The 
pressure  of  the  air  on  the  water  in  the  vessel  pushes  the 
liquid  up  into  the  bottle  and  holds  it  there. 

It  is  the  pressure  of  air  which  also  drives  water  through 


50 


NATURAL  PHILOSOPHY. 


a  glass  tube  or  a  straw  when,  one  end  being  in  the  liquid, 
the  lips  are  applied  to  the  other  end.  All  that  the  lips 
do  is  to  take  the  air  out  of  the  tube  above  the  water. 

How  high  a  column  of  water  will  the  pressure 
of  the  air  sustain  ? — The  water  will  be  sustained  to  a 

height  of  about  34  feet 
by  this  pressure  of  the  air. 
How  high  a  column 
of  mercury? — A  col- 
umn of  mercury  about 
30  inches  high  will  weigh 
as  much  as  a  column  of 
water  34  feet  high  if  they 
are  of  the  same  size. 
Therefore,  a  column  of 
mercury  only  30  inches 
high  will  be  supported 
by  the  pressure  of  the  air 
Can  we  prove  this 
by  experiment  ?— The 
pictures  show  how  this 
can  be  easily  proved  by 
experiment.  A  glass  tube 
more  than  30  inches 
long  is  taken.  One 
end  of  it  is  closed,  the 
other  open.  It  is  first 
filled  with  mercury,  and 
then  the  open  end  is 
shut  with  the  finger, 
while  the  tube  is  turned,  closed  end  upward,  as  shown  in 
Fig.  20.  The  open  end  is  then  put  into  a  dish  of  mer- 
cury nnd  the  finger  taken  away.  The  mercury  will  no 


• 


GASES. 


51 


longer  quite  fill  the  tube,  but  will  sink  so  as  to  leave  the 
upper  part  empty,  as  shown  in  Fig.  21.  The  air-press- 
ure supports  this  column  of  mercury,  and  the  height 
measures  about  30  inches. 

How  much  pressure  does  the  atmosphere  exert 
upon  a  square  inch  ? — A  column  of  mercury  one  inch 
square  and  thirty   inches   high  will  weigh    15   Ibs.     To 
balance  15  Ibs.  of  mercury  in  the  tube,  or  to  keep  it  from 
F!s-  21.  falling  out,   would   need 

another  pressure  of  15 
Ibs.,  and  since  the  air 
does  this  we  know  that  it 
must  be  exerting  a  press- 
ure of  15  pounds  to  the 
square  inch. 

Air  is  lighter  than  the 
lightest  down,  but  reach- 
ing above  us  to  a  height 
of  many  miles,  the  quan- 
tity in  all  must  be  im- 
mense. It  covers  every 
thing  upon  the  earth,  and 
presses  upon  each  square 
inch  of  the  surface  of 
every  thing  with  a  force 
of  15  Ibs.  The  total 
pressure  upon  each  oiie 
of  our  bodies  is  said  to  be 
about  20,000  Ibs.  We 
are  able  to  bear  this  great  pressure  without  feeling  it, 
because  it  is  so  exactly  equal  in  all  directions,  and  because 
every  point  on  the  body,  bearing  its  own  share,  divides 
the  labor  so  justly  that  no  point  is  burdened. 


52  NATURAL  PHILOSOPHY. 

Is  the  pressure  of  the  atmosphere  always  alike  ? 

— By  watching  the  column  of  mercury  in  the  tube,  Fig. 
21,  it  will  be  found  to  be  higher  at  some  times  than  at 
others.  This  must  be  because  the  pressure  of  the  atmos- 
phere is  not  always  the  same.  It  is  sometimes  a  little 
more  than  15  Ibs.  to  the  inch  and  sometimes  a  little 
less. 

How  does  the  pressure  vary  with  the  weather  ? 
— In  bright,  clear  weather  the  atmosphere  is  heavier  than 
when  storms  or  clouds  prevail.  Hence  the  column  of 
mercury  will  be  higher  in  fair  weather  than  in  foul 
weather;  and  by  watching  the  changes  in  the  height  of 
the  mercury  column  \ve  may  judge  something  of  what  the 
character  of  the  weather  is  to  be. 

How  does  the  pressure  vary  with  the  height  ?— 
At  the  level  of  the  sea  the  column  of  mercury  stands  30 
inches  high.  A  gentleman  by  the  name  of  Pascal,  in 
France,  with  a  tube  and  cistern  of  mercury,  traveled  up  a 
mountain-side  to  find  what  effect,  if  any,  would  be  produced 
upon  the  column.  As  he  climbed  the  mountain  higher 
and  higher,  he  found  that  the  mercury  sank  lower  and 
lower  in  the  tube.  We  learn  from  his  experiment  that  the 
pressure  of  the  atmosphere  is  less  as  the  height  is  greater. 

May  we  not  know  this  without  experiment  ? — 
Indeed,  no  experiment  is  needed  to  prove  this,  for  it  is 
very  clear  that  when  going  up  we  leave  a  part  of  the 
atmosphere  below  us,  and  there  being  then  less  above  us, 
the  pressure  exerted  mu^t  be  less. 

What  is  the  Barometer? — Now  the  barometer  is  an 
instrument  which  shows  the  changes  in  the  pressure  of 
the  atmosphere.  It  consists  of  a  tube  with  a  cistern  of 
mercury,  like  that  shown  in  Fig.  21,  placed  in  a  frame- 
work of  wood  or  metal  which  protects  it  from  injury.  A 


GASES. 


53 


scale  placed  behind  the  tube  shows  the  height  of  the 
mercury  column. 

How  does  this  instrument  foretell  changes  in 
the  weather  ?— By  the  motion  of  the  mercury  up  or 
down  we  may  judge  something  of  the  future  character  of 
the  weather.  If  we  see  the  mercury  rising,  we  may  expect 
fair  weather :  if  the  mercury  falls,  we  may  expect  foul 
weather. 

How  is  it  used  to  find  the  height  of  mountains  ? — 
From  the  sea-level  to  the  top  of  Mont  Blanc  is  a  height 
of  about  15,000  feet.  In  going  to 
the  summit,  the  mercury  column 
falls  about  15  inches.  From  this, 
and  other  observations  like  this, 
it  appears  that  the  mercury  sinks 
about  one  inch  for  every  thousand 
feet  the  barometer  is  taken  upward. 
To  get  the  height  of  a  mountain, 
then,  we  may  count  1,000  feet  for 
every  inch  through  which  the  mer- 
cury falls  in  being  carried  to  the 
summit. 

This  rule  will  not  give  the  exact 
height,  owing  to  things  which  we 
will  not  now  try  to  explain,  but 
when  all  things  are  taken  into 
account  in  making  the  calculations, 
the  height  of  a  mountain  may  be 
calculated  by  the  barometer  per- 
haps more  easily  and  exactly  than  | 
by  any  other  method. 

Describe  the  lifting-pump. — In  this  very  common 
and  useful  instrument  the  pressure  of  the  atmosphere  ia 


I  c 


NATURAL  PHILOSOPHY. 


made  to  lift  water  from  a  well  or  cistern.  Its  parts  and 
their  action  are  very  wrell  shown  in  Fig.  22.  It  consists 
of  two  cylinders,  one  above  the  other,  the  upper  one,  6Y, 
being  often  much  larger  than  the  other,  c,  which  reaches 
down  into  the  well.  Where  these  cylinders  join  each 
other  there  is  a  little  door,  or  valve,  which  opens  upward, 
and  will  allow  water  to  go  up,  but  will  not  let  it  go 
down  again.  In  the  upper  cylinder  there  is  a  piston 
which  may  be  lifted  or  pushed  down  by  the  handle  of 
the  purnp.  In  this  piston  there  is  a  valve,  or,  it  may  be, 
more  than  one,  which  opens  upward. 

Explain  the  action  of  the  pump. — By  the  first 
strokes  of  the  piston  the  air  is  taken  out  of  the  cylinders, 
and  then  the  pressure  of  the  atmos- 
phere upon  the  water  in  the  well 
pushes  the  water  up  into  the  pump 
just  as  it  will  push  water  up  into  a 
straw  or  other  tube  when  the  air  is 
drawn  out  at  the  top  by  applying 
the  lips.  After  the  air  has  been  all 
taken  out,  the  water  will  till  the 
pump  full  to  the  spout,  and  then 
every  time  the  piston  is  raised  it  will 
lift  a  portion  of  water  out  at  the 
spout,  while  more  is  pushed  in  at  the 
bottom  by  the  air  to  take  its  place. 

Describe  the  force-pump. — Fig. 
23  shows  a  kind   of  pump  which  is 
g_  used   for  throwing  water  to  greater 


heights.    It  is  called  t\\Q  force-pump. 

The  spout  is  at  the  bo;  torn  of  the 
upper  cylinder  instead  of  near  the  top  of  it,  as  in  the  lift- 
ing-pump, and  instead  of  having  any  valve  in  the  piston 


GASES. 


there  is  one  opening  into  the  spout.     In  other  respects  it 
is  like  the  lifting-pump. 

Explain  its  action. — When  the  piston  is  lifted  the 
atmosphere  pushes  water  up  into  the  upper  cylinder. 
When  the  piston  is  pushed  down  the  water  is  pushed 
through  the  valve  into  the  spout.  This  spout  may  reach 
even  to  the  top  of  a  house,  and  the  water  will  go  higher 
by  each  stroke  of  the  piston  until  it  reaches  the  top  and 
runs  over.  A  jet  of  water  would  be  thrown  out  by  each 
downward  stroke  of  the  piston,  but  if  a  steady  stream,  is 
wanted,  the  spout  leads  into  an  air-chamber  where  the 
water  condenses  the  air.  This  condensed  air  exerts  a 
steady  pressure  on  the  water  in  the  chamber  and  throws 
it  out  in  a  steady  stream. 

The  fire-engine  is  a  form  of  force-pump.  In  the  steam 
fire-engine  the  piston  is  moved  by  the  power  of  steam. 

What  is  the  siphon?— In  Fig.  24,  the  tub*  from 
which  the  water  appears  to  be  running  is  a  siphon.  The 
instrument  is  used  to  pass  a 
liquid  from  one  vessel  to  an- 
other. The  siphon  is  never 
any  thing  more  than  a  bent 
tube,  one  arm  being  longer 
than  the  other. 

How  is  it  used?— When 
the  siphon  is  to  be  used,  it 
must  first  be  tilled  with  water, 
and  then  the  end  of  the  long 
arm  is  closed  with  the  finger, 
while  the  short  arm  is  put  into 
the  liquid  in  the  vessel.  The 
moment  the  finger  is  removed,  the  liquid  will  begin  to 
flow  up  the  short  arm,  over  the  bend  and  out  at  the  end 


56  NATURAL   PHILOSOPHY. 

of  the  long  arm  ;  nor  will  it  stop  running  until  the  end 
of  the  short  arm  is  uncovered,  or  until  the  liquid  is  as 
high  in  the  second  vessel  as  in  the  one  from  which  it  runs. 

Explain  its  action. — It  often  at  first  seems  a  mystery 
why  the  liquid  should  run  upward  through  the  short  arm 
and  out  from  the  other ;  but  we  shall  see  that  the  forces 
that  push  it  in  that  direction  are  stronger  than  those  that 
push  the  other  way. 

To  push  it  out  through  the  long  arm  there  is,  first, 
the  weight  of  the  liquid  in  that  arm,  and,  second,  the 
pressure  of  the  air  on  the  water  in  the  vessel. 

To  keep  it  in,  there  is,  first,  the  weight  of  the  water  in 
the  short  arm,  and,  second,  the  pressure  of  the  air  upward 
against  the  water  at  the  end  of  the  long  arm. 

Now  the  first  tvvo  forces  are  stronger  than  the  last  two, 
because  the  weight  of  the  water  in  the  long  arm  is  greater 
than  of  that  in  the  short  arm  ;  and  the  water  runs  in 
the  direction  of  the  greater  force. 


MOTION. 


Can  bodies  move  themselves?  —  Animals  can 
move  from  place  to  place,  because  they  have  the  power 
of  will,  and  their  bodies  must  obey  it ;  but  that  a  book  or 
a  block  of  stone  should  of  its  own  accord  move  out  of  its 
place,  we  pronounce  to  be  impossible.  Such  bodies  of 
matter  can  move  only  as  they  are  either  pushed  or  pulled. 
The  ship,  for  example,  is  pushed  along  by  wind  :  a  train 
of  cars  is  pulled  along  by  a  steam-engine.  A  stone  falls 
to  the  ground  because  it  is  pulled  down  by  the  attraction 
of  the  earth,  and  the  smoke  rises  because  it  is  pushed  up- 
ward by  the  heavier  air. 

A  body  at  rest  would  rest  forever  if  it  were  neither 
pushed  nor  pulled  by  some  force  outside  of  itself. 

Can  bodies  stop  themselves  ?— A  moving  ship  will 
not  stop  suddenly  when  the  sails  are  taken  down.  The 
water  finally  stops  it ;  no  one  ever  thinks  of  saying  that 
tho  ship  stops  itself.  A  train  of  cars  moves  along  some  dis- 
tance after  the  steam  has  been  cut  off:  it  is  finally  stopped 
by  the  friction  of  the  wheels  as  they  rub  upon  their  axles 
and  upon  the  track,  together  with  other  obstacles  which 
it  meets ;  it  does  not  stop  itself.  A  horse  and  his  rider 
are  moving  along  together ;  let  the  horse  suddenly  stop, 
and  his  rider  is  plunged  over  his  head ;  the  man  can  not 
stop  himself. 

We  learn  from  these  observations  that  a. body  in  motion 

r 


58  NATURAL  PHILOSOPHY. 

would  move  forever  if  it  were  not  stopped  by  some  force 
outside  of  itself. 

Can  a  body  change  the  direction  of  its  mo- 
tion ? — When  a  ball  is  struck  with  a  bat,  it  flies  in  the 
direction  of  the  blow,  and  in  no  other,  unless  turned 
aside  by  some  other  force. 

The  same  thing  appears  to  be  true  of  all  other  motions, 
for  who  ever  saw  a  moving  body  suddenly  of  its  own  ac- 
cord start  off  in  another  direction  !  A  body  in  motion 
would  move  forever  in  a  straight  line  unless  turned  aside 
by  some  force  outside  itself. 

What  is  the  first  law  of  motion  ? — We  may  now 
state,  in  very  few  words,  what  we  have  thus  far  learned 
about  motion,  as  follows : 

A  body  at  rest  would  rest  forever,  or  if  in  motion  would 
move  forever  in  a  straight  line,  unless  kept  from  doing  so 
by  some  force  outside  of  itself. 

This  principle  is  called  the  first  law  of  motion. 

Why  then  does  not  a  stone  move  in  a  straight 
line  when  thrown  from  the  hand? — When  a  stone 
is  thrown  in  a  horizontal  direction,  we  find  that  instead 
of  going  along  in  that  direction,  it  very  soon  flies  lower 
and  lower,  until  at  length  it  strikes  the  ground.  It  would 
move  in  a  straight  line  if  the  attraction  of  the  earth  did 
not  pull  it  to  the  ground. 

Are  other  motions  caused  by  more  than  one 
force? — If  the  wind  blows  while  the  rain  falls  we  see 
the  drops  coming  obliquely  down  to  the  ground.  Gravi- 
tation alone  would  bring  them  vertically  through  the  air, 
but  the  wind  at  the  same  time  pushes  them  sidewise  ; 
their  motion  is  due  to  these  two  forces. 

Now,  the  more  examples  of  motion  we  examine,  the 
more  certain  we  become  that  the  motions  of  bodies  are 


MOTION.  50 

generally  caused  by  two  or  more  forces  acting  upon  them 
at  the  same  time. 

Does  each  force  produce  as  much  effect  as  if  it 
acted  alone  ? — Here  is  an  experiment  that  any  one  can 
try  for  himself  while  studying  this  subject.  Put  a  ball  at 
one  corner  of  a  table.  You  may  snap  it  with  the  fingers  of 
one  hand  and  make  it  roll  along  the  side  of  the  table,  and 
if  you  afterward  snap  it  with  the  fingers  of  the  other 
hand  you  may  make  it  roll  across  the  end;  but  if  you 
skillfully  snap  it  with  the  fingers  of  both  hands  at  once,  it 
will  follow  neither  the  side  nor  the  end,  but  you  will  see 
it  dari  obliquely  across  to  the  opposite  corner. 

If  one  hand  would  roll  the  ball  the  whole  length  of  the 
table  in  one  second,  and  if  the  other,  would  roll  it  across 
the  end  in  one  second,  then,  when  both  hands  were  used 
at  once,  the  ball  will  roll  to  the  opposite  corner  in  exactly 
one  second.  But  to  get  to  this  opposite  corner,  the  ball 
must  go  the  whole  length  and  the  whole  width  of  the  table 
both  at  once;  so  that  each  force  causes  just  as  much  mo- 
tion in  the  second  of  time  as  if  it  were  acting  alone. 

Give  another  example. — The  swift  motion  of  a  can- 
non-ball is  caused  by  the  explosion  of  gunpowder,  but 
gravitation  is  at  the  same  time  pulling  the  ball  down 
toward  the  ground.  If  the  ball  is  shot  in  a  horizontal 
direction  it  will  strike  the  ground  at  the 'same  time  it 
would  if  dropped  from  the  mouth  of  the  gun.  The  force 
of  gravitation  pulls  the  ball  downward  through  the  same 
distance  while  it  is  moving  horizontally  as  it  would  in  the 
same  time  if  falling  vertically. 

Suppose  the  ball  shot  directly  upward. — A  ball 
will  fall  about  16  feet  in  the  first  second  after  it  starts. 
Now  suppose  a  ball  shot  directly  upward,  and  that  the 
force  of  the  powder  would  be  strong  enough  to  send  it  up 


00  NATURAL  PHILOSOPHY. 

100  feet  in  the  first  second  :  the  ball  will  only  rise  84  feet. 
The  attraction  of  gravitation  which  would  make  it  fall  16 
feet  if  it  were  not  for  the  powder,  will  cut  off  just  16  feet 
from  its  ascent. 

What  is  the  second  law  of  motion?— From  such 
facts  as  these  we  infer  that : 

A  force  will  cause  the  same  amount  of  motion,  and  in 
1he  same  direction,  whether  the  body  it  acts  upon  be  at  rest 
or  already  in  motion. 

This  principle  is  called  the  second  law  of  motion. 

What  is  meant  by  action  and  reaction  ? — He  who 
strikes  the  table  with  his  hand  gets  a  blow  from  the  table 
in  return,  as  he  very  well  knows  by  the  pain  it  occasions 
when  the  blow  is  hetivy.  The  hand  acts  upon  the  table 
and  the  table  reacts  upon  the  hand.  Whenever  two 
bodies  act  upon  each  other,  the  effect  of  one  of  them  is 
called  action,  that  of  the  other  is  called  reaction. 

A  bullet  may  perhaps  fracture  a  stone  against  which  it 
is  fired,  but  the  bullet  will  be  flattened,  showing  that  the 
stone  has  returned  the  blow.  The  bullet  acts  upon  the 
stone  and  the  stone  reacts  upon  the  bullet. 

Are  action  and  reaction  in  the  same  direction  ? — 
When  a  book  lies  upon  the  table  it  presses  downward,  but 
the  table  is  at  the  same  time  pressing  upward  to  keep  the 
book  from  going  to  the  floor.  Action  and  reaction  must 
always  be  in  opposite  directions. 

Which  is  the  stronger  ?— Take  the  case  of  the  book 
on  the  table :  the  action  of  the  book  downward,  or  its 
pressure,  is  just  equal  to  its  weight.  If  the  table  should 
react  with  a  force  (/reater  than  the  weight  of  the  book,  it 
would  throw  the  book  upward ;  if  with  less  force,  the 
book  would  be  able  to  break  through  it :  it  can  be 
neither  greater  nor  less,  because  the  book  is  at  rest.  In 


MOTION.  61 

every  other  case  as  well  as  this,  action  and  reaction  are 
equal. 

What  is  the  third  law  of  motion? — From  what  has 
just  now  been  said  we  may  gather  this  general  statement: 

Every  action  must  be  followed  ly  an  opposite  and 
equal  reaction. 

This  principle  is  called  the  third  law  of  motion. 

What  is  an  impulsive  force? — When  a  bullet  is 
shot  from  a  gun,  the  force  of  the  gunpowder  acts  upon  it 
only  for  a  single  moment  when  it  starts.  A  force  which 
acts  for  a  moment  only  is  called  an  impulsive  force. 

Other  examples  are  common  enough.  When  a  ball  is 
hit  with  a  bat,  the  force  of  the  blow  is  spent  upon  the  ball 
in  an  instant.  And  at  the  moment  when  a  stone  leaves 
the  hand  that  throws  it,  the  force  of  the  hand  is  spent. 
Both  of  these  are  impulsive  forces. 

What  do  we  notice  about  the  motion  caused  by 
impulsive  forces  ? — We  notice  that  the  motions  produced 
by  these  impulsive  forces  are  all  alike  in  one  thing  at 
least:  the  velocity  is  greatest  at  the  beginning.  The 
speed  of  the  bullet  is  greatest  at  the  moment  when  it 
leaves  the  gun,  and  grows  less  and  less  until  it  is  stopped 
entirely.  And  in  the  case  of  the  ball  struck  with  a  bat, 
and  of  the  stone  thrown  from  the  hand,  motion  is  swiftest 
at  the  beginning  and  gradually  grows  slower. 

Why  do  these  motions  grow  slower  ? — It  would 
not  be  so  if  it  were  not  for  the  resistance  of  the  air.  The 
resistance  of  the  air  which  hinders  the  snow-flakes  so  that 
they  can  not  fall  like  rain-drops  or  hail-stones,  also  hinders 
the  motion  of  every  thing  else. 

Cut  a  leaf  of  paper  into  pieces  an  inch  in  length  and 
half  as  wide  ;  toss  them  upward  into  the  quiet  air  of  the 
room  aniL^vvatch  their  slow  and  curious  motions  to  the 


and  wa 


32  NATURAL  PHriX)SOPHY. 

floor :  if  it  were  not  for  the  air  which  hinders  them  they 
would  fall  like  bullets. 

The  air  hinders  the  motion  of  heavy  bodies,  and  the 
faster  they  move  the  more  it  will  affect  it.  Even  the 
motion  of  cannon-balls  is  rapidly  lessened  by  the  resist- 
ance of  the  air. 

Now  if  there  were  no  air  nor  other  resistances  the  mo- 
tion caused  by  an  impulsive  force  would  not  grow  less. 
The  moving  body  would  pass  over  equal  distances  in  equal 
times',  or  in  other  words,  its  motion  would  be  uniform. 

What  is  a  constant  force  ?— The  force  of  the  hand 
which  tosses  a  stone  upward  into  the  air  is  an  impulsive 
force,  but  gravitation  which  brings  it  down  again  is  not. 
Never  for  a  single  instant  does  gravitation  cease  to  act 
upon  the  falling  stone,  and  on  this  account  it  is  called  a 
constant  force.  A  constant  force  is  one  whose  action  is 
all  the  time  alike. 

What  do  we  notice  about  the  motion  it  pro- 
duces?—The  motion  of  a  falling  body  is  swifter  and 
swifter  the  farther  it  falls.  This  is  true  not  only  of  mo- 
tion caused  by  gravitation,  but  of  motion  caused  by  any 
constant  force  whatever ;  the  velocity  increases  while  the 
force  is  acting. 

How  do  the  air  and  other  resistances  affect 
this  motion?— In  this  case  also  the  resistance  of  the  air 
hinders  the  motion,  and  it  hinders  it  more  and  more  as  the 
velocity  is  greater.  In  fact,  the  motion  of  a  body  falling 
from  a  great  height  may  become  so  swift  that  the  resistance 
of  the  air  will  be  as  strong  as  the  force  of  gravitation  itself, 
and  after  that  moment  the  motion  will  be  uniform. 

It  is  just  so  with  a  train  of  cars.  The  power  of  the 
steam  starts  it  and  for  a  little  time  makes  it  go  faster  and 
faster,  but  the  motion  soon  becomes  uniform  because  the 


i)  been vi so 


MOTION.  63 

many  resistances  which  the  train  meets  soon  equals  the 
power  of  the  steam. 

The  motion  ot  a  sail-boat  increases  at  first,  luit  very 
soon  the  resistance  of  the  water  becomes  so  strong  that  it 
needs  the  whole  force  of  the  wind  to  overcome  it,  and  after 
that  the  boat  sails  on  at  a  uniform  rate. 

In  -what  kind  of  a  path  will  an  arrow  go  ?— If  an 
arrow  is  shot  from  the  bow  directly  upward,  it  will  go  up  in 
a  straight  line  and  its  motion  back  again  will  also  be  in  a 
straight  line.  If,  however,  the  arrow  be  fired  in  any  other 
direction  its  path  will  be  a  curve  instead  of  being  straight. 

Why  will  the  path  of  the  arrow  be  curved? — It 
is  easy  to  see  that  there  are  two  forces  acting  on  the  arrow. 
There  is,  first,  the  force  of  the  bow  which  sends  it  forward, 
and  then,  second,  the  force  of  gravitation  which  pnlls  it 
toward  the  ground.  The  bow  would  send  it  in  a  straight 
line,  but  gravitation  is  all  the  time  pulling  it  down  out  of 
that  line.  The  arrow  obeys  both  of  these  forces  at  once, 
going  forward  and  downward  at  the  same  time,  its  direc- 
tion changing  a  little  all  the  time.  For  this  reason  the 
path  of  the  arrow  is  curved. 

In  any  case,  if  a  body  moves  in  a  curved  path  it  is  being 
acted  on  by  two  forces,  and  one  of  these,  at  least,  must 
constantly  act. 

Give  another  example. — We  may  make  an  easy  ex- 
periment to  illustrate  this  statement  more  fully.  Tie  a 
string  to  the  stem  of  an  apple  and  make  the  apple  swing 
around  the  hand  in  a  circle.  You  can  feel  the  apple 
pulling  as  if  struggling  to  get  away  from  the  hand,  and 
should  you  let  go  your  hold  of  the  string  the  apple  would 
dart  off  in  a  straight  line  in  just  whatever  direction  it 
happened  at  the  moment  to  be  going. 

What  two  forces  make  the  apple  move  in  the 


64  NATURAL   PHILOSOPHY. 

circle? — There  is  one  force,  we  notice,  which  is  trying  to 
move  the  apple  in  a  straight  line,  and  the  string  is 
another  force  which  is  pulling  it  out  of  that  line  every 
moment, 

By  these  two  forces  acting  together  the  apple  is  moved 
in  a  circle. 

What  are  the  centrifugal  and  centripetal  forces  ? 
— Now  one  of  the  two  forces  by  which  curved  motion  is  pro- 
duced has  been  called  the  centrifugal  force,  and  the  other 
the  centripetal  force.  The  one  which  would  send  the 
body  away  in  a  straight  line  is  the  centrifugal  force ;  that 
which  pulls  it  out  of  the  straight  line  is  the  centripetal 
force. 

What  familiar  examples  of  the  action  of  these 
forces? — The  stone  in  a  sling,  at  the  moment  when  it  is 
set  at  liberty,  darts  off  in  a  line  as  straight  as  the  path  of 
an  arrow  or  a  bullet :  but  before  it  is  set  at  liberty,  the 
sling-cord  pulls  it  out  of  that  line  and  keeps  it  moving  in 
a  circle. 

A  wet  mop,  made  to  turn  swiftly  on  its  handle  as  an 
axis,  throws  the  water  in  all  directions  and  soon  dries 
itself.  It  is  the  centrifugal  force  which  sends  the  water 
away.  And  this  illustrates  what  we  sometimes  see  among 
animals.  Sheep,  for  example,  in  wet  weather  throw  the 
water  off  themselves  by  shaking  their  fleeces  in  a  kind  of 
half  rot^ary  motion.  Water-dogs  on  coming  to  land  dry 
themselves  in  the  same  way. 

"A  loaded  stage-coach  running  south  and  turning  sud- 
denly to  the  east  or  west,  strews  its  passengers  on  the 
south  side  of  the  road.  A  man  on  horseback  when  turn- 
ing a  corner  leans  much  toward  the  corner  in  order  to 
overcome  the  centrifugal  force  which  would  throw  him 
away  from  it." 


MOTION.  65 

A  carriage-wheel  turning  swiftly  often  throws  the  dirt 
in  straight  lines  from  its  circumference.  In  the  same 
way,  were  it  not  for  the  attraction  of  gravitation,  all 
bodies  on  the  face  of  the  earth  would  be  thrown  out  into 
the  heavens  by  the  centrifugal  force  due  to  the  rotation 
of  the  eartli  upon  its  axis. 

The  earth  is  moving  in  an  orbit  which  is  almost  a  cir- 
cle, the  diameter  of  which  is  about  190,000,000  miles,  and 
it  is  going  at  the  rate  of  about  68,000  miles  an  hour.  At 
every  moment  during  this  wonderful  journey  the  earth  is 
struggling  to  fly  away  in  a  straight  line,  but  the  powerful 
attraction  of  the  sun  is  the  strong  arm  which  constantly 
pulls  it  out  of  this  line  into  the  graceful  curve  through 
which  it  flies. 


VIBRATIONS. 


Describe  the  pendulum. — In  Fig.  25  we  notice  a 
ball  B  hung  from  a  fixed  point  A  by  means  of  a  cord. 
This  ball  represents  a  pendulum  : 
any  body  hung  from  a  fixed  point, 
under  which  it  may  swing  from 
side  to  side,  backward  and  for- 
ward, is  a  pendulum. 

If  such  a  bull  were  pulled  aside 
and  then  dropped  !t  would  swing 
for  a  long  time.  You  can  easily 
try  the  experiment  by  hanging  an 
'-  apple  in  the  same  way  and  mak- 
ing it  swing. 

What  is  meant  by  vibra- 
tion and  amplitude  ?— If  the  pendulum  (Fi>.  25)  h 
lifted  to  C  it  will  swing  to  D,  a  point  almost  as  far  on  the 
other  side,  and  then  return.  It  will  keep  on  moving  back 
and  forth  in  tin's  arc  until  the  resistance  of  the  air  finally 
stops  it  at  B,  the  place  from  which  it  first  started.  Its 
motifni  from  one  end  of  its  arc  tj  the  other  is  called  a 
vibration,  and  the  distance  from  one  end  of  its  arc  to  the 
other  is  called  its  amplitude. 

Does  the  time  of  one  vibration  depend  upon 
amplitude? — The  distance  through  which  the  pendulum 
swings  makes  very  little  difference  in  the  time  it  takes  to 


VIBRATIONS 


67 


pass  through  it ;  in  other  words,  a  pendulum  will  swing 
through  a  long  arc  just  as  quickly  as  through  a  short  one. 
The  reason  that  the  long  journey  is  made  in  the  same 
time  as  the  short  one  is  this :  the  longer  the  arc  the  steeper 
are  its  ends,  and  on  this  account  the  swifter  the  pendulum 
will  fall. 

Does  the  time  of  one  vibration  depend  upon  the 
•weight  of  the  ball  ? — It  is  another  curious  property  of 
the  pendulum  that  whether  it  be  made  of  lead  or  of  wood 
or  of  other  material,  it  will  make  its  vibration  in  the 
same  time.  Its  weight,  and  we  may  add,  the  material  of 
which  it  is  made,  makes  no  difference  in  the  time  of  one 
vibration. 

Does  the  time  of  one  vibration  depend  upon  the 
size  of  the  pendulum  ? — Nor  does  the  size  of  the  pen- 
dulum make  any  difference  in  the  time  of  one  vibration. 
Of  course  the  resistance  of  the  air  will  be 
more  on  a  large  ball  than  on  a  small  one, 
and  on  that  account  a  large  pendulum 
will  not  continue  to  vibrate  as  long  as  a 
small  one,  but  they  will  swing  from  one 
end  of  the  arc  to  the  other  in  the  same 
time,  while  the  motion  does  continue,  no 
matter  how  much  they  may  differ  in  size. 

Upon  what  does  the  time  of  one 
vibration  depend? — But  if  we  take 
pendulums  of  different  lengths,  as  shown 
in  Fig.  2G,  we  shall  find  the  longest  one 
vibrating  slowest.  In  all  cases  the  long- 
est pendulum  needs  the  longest  time  to 
make  one  vibration.  The  time  of  one 
vibration  depends  altogether  upon  the 
length  of  the  pendulum. 


Fig.  26. 


jgg  NATURAL  PHILOSOPHY. 

What  is  the  law?— If  the  pendulum  P  (Fig.  26)  is 
just/bwr  times  as  long  as  another,  P',  we  shall  tind  by 
trying  the  experiment  that  it  takes  just  twice  the  time  it 
does  the  other  to  make  a  vibration.  We  notice  that : 

The  time  of  one  miration  is  in  proportif/i  to  the  (square 
root  of  the  Length  of  the  pendulum. 

If  then  one  pendulum  is  9  times  the  length  of  another, 
it  will  take  it  3  times  as  long  to  vibrate  once. 

The  length  of  a  pendulum  that'  will  vibrate  in  one 
second  is  about  39.1  inches:  a  pendulum  \  of  that  length 
would  vibrate  in  £  a  second  according  to  the  law,  since 
the  square  root  of  \  is  \. 

How  is  the  pendulum  used  to  measure  time  ? — 
Now  if  we  know  the  time  it  takes  a  pendulum  to  make 
one  vibration,  we  may  measure  any  length  of  time  by 
simply  counting  the  number  made.  The  resistance  of  air 
would  however  soon  stop  the  swinging,  and,  even  if  it  did 
not,  the  counting  of  the  vibrations  would  be  a  tedious 
task.  Ingenious  men  have  overcome  these  difficulties  by 
inventing  the  clock,  by  which  people  are  everywhere  able 
to  measure  time. 

Briefly  describe  its  action. — In  this  instrument  a 
weight  or  a  spring  keeps  a  set  of  wheels  in  motion,  and 
these  wheels  keep  the  pendulum  vibrating  and  at  the 
same  time  register  the  number  of  vibrations  it  makes,  by 
making  an  index  or  hand  point  to  the  divisions  of  a  grad- 
uated circle. 

Explain  its  action  more  fully.— In  looking  at  the 
interior  of  a  common  clock,  which  is  the  best  and  per- 
haps the  only  way  any  one  can  clearly  learn  its  action, 
we  find  that  a  pendulum  is  so  connected  with  a  toothed 
wheel  that  at  the  end  of  every  two  vibrations  it  allows 
one  tooth  to  escape.  If  the  pendulum  vibrates  twice  a 


VIBRATIONS.  69 

second  it  allows  one  tooth  to  escape  at  the  end  of  each 
second,  and  if  there  are  sixty  teeth  on  the  wheel  it  will 
turn  around  just  once  in  sixty  seconds  or  a  minute.  To 
the  axis  of  this  wheel  thg  second-hand  of  the  clock  is 
fixed. 

This  wheel  is  connected  with  another  that  turns  once 
around  in  an  hour,  and  to  the  axis  of  this  one  the  minute- 
hand  is  fastened. 

There  is  still  another  wheel  in  the  set  which  can  turn 
once  around  in  twelve  hours,  and  to  tho  axis  of  this  the 
hour-hand  is  fastened. 

How  does  a  watch  differ  from  a  clock  ?— A  watch 
differs  from  a  clock  in  having  a  vibrating  wheel,  called 
the  balance-wheel,  instead  of  a  pendulum.  The  vibration 
of  the  balance-wheel  allows  one  tooth  of  a  wheel  to  pass 
just  as  the  pendulum  does  in  the  clock,  and  the  number 
of  beats  is  recorded  in  the  same  way. 

What  are  chronometers  ?  —  Time-keepers  of  the 
most  wonderful  perfection  have  been  made  for  the  pur- 
pose of  telling  the  longitude  of  a  ship  at  sea,  and  for  other 
purposes  where  great  accuracy  is  required.  They  are 
called  chronometers. 

Concerning  their  perfect  action  Arnott  says:  After 
months  spent  in  a  passage  from  South  America  to  Asia, 
my  pocket  chronometer,  with  others  on  board,  announced 
one  morning  that  a  certain  point  of  land  was  then  bearing 
east  from  the  ship  at  a  distance  of  fifty  miles  ;  and  in  an 
hour  afterwards,  when  a  mist  had  cleared  away,  the 
looker-out  on  the  mast  gave  the  joyous  call,  "  Land  ahead," 
verifying  the  report  of  the  chronometers  almost  to  a  mile 
after  a  voyage  of  thousands. 

The  method  of  using  a  watch  to  tell  the  longitude  of  a 
place  on  the  earth  may  be  found  explained  in  astronomy. 


TO  NATURAL  PHILOSOPHY. 

What  other  use  may  be  made  of  the  pendulum  ? 

— The  pendulum  lias  been  used  to  determine  the  shape  of 
the  earth. 
Describe  the  experiment  with  a  vibrating  cord. 

— Let  a  small  cord  fastened  at  one  end  pass  over  two 

Fig.  27. 


bridges  upon  which  it  rests  and  be  stretched  by  a  heavy 
weight  hung  at  the  other  end  (Fig.  27).  Then  if  a  violin 
bow  be  drawn  across  the  cord,  or  if  a  person,  taking  hold 
of  its  middle  point,  pull  it  aside  and  let  it  go  again,  it  will 
swing  back  and  forth  so  swiftly  that  its  motions  can  not  he 


VIBRATIONS.  71 

counted,  and  it  will  look  like  a  gauzy  spindle,  as  the  picture 
represents  it. 

What  is  meant  by  vibration  and  amplitude  ? — 
The  continued  motion  of  the  cord  back  and  forth  is  called 
vibratifm.  But  when  we  speak  of  a  vibration,  or  a  single 
vibration,  we  mean  the  {notion  from  one  aide  to  the  other 
and  lack  again  to  the  starting-point. 

The  distance  from  one  side  to  the  other,  that  is,  the 
distance  through  which  any  point  of  the  cord  travels,  is 
called  its  amplitude  of  vibration. 

Can  the  number  of  vibrations  be  counted  ?— The 
motion  of  the  cord  is  so  very  rapid  that  all  we. can  see 
when  it  vibrates  is  a  gauze-like  swelling  of  its  middle 
parts ;  and  yet  it  is  possible  to  find  out  exactly  how 
many  vibrations  it  makes  in  a  second. 

If  the  cord,  like  those  of  a  violin  or  piano,  gives  a  sound 
when  it  vibrates,  the  number  of  vibrations  may  be  regis- 
tered by  the  syren :  for  the  number  will  be  the  same  as 
the  number  of  air-puffs  which  escape  from  that  instru- 
ment when  it  makes  a  sound  of  the  same  pitch  as  that 
made  by  the  cord,  and  the  number  of  air-puffs  is  registered 
by  the  "  hands  "  upon  the  upper  part  of  the  instrument,  as 
seen  in  Fig.  31. 

If  the  vibrations  of  the  cord  do  not  produce  sound,  yet 
the  number  made  in  one  second  may  be  very  exactly 
shown  by  the  aid  of  electricity.  We  will  not  attempt  to 
describe  the  instrument  now.  See  Text-look  of  Phi- 
losophy, p.  151. 

Does  the  rapidity  of  vibration  depend  upon  the 
length  of  the  cord  ? — When  two  cords  are  taken,  one 
twice  as  long  as  the  other,  but  alike  in  every  other  re- 
spect, it  is  found  by  experiment  that  the  long  one  will 
vibrate  onlv  one  half  as  fast  as  the  other.  In  this  cafe 


72  NATURAL  PHILOSOPHY. 

the  number  of  vibrations  in  a  second  is  inversely  as  the 
length  of  the  cord. 

This  is  also  true  of  all  other  cases.  If  one  string  is,  for 
example,  one  tenth  as  long  as  anothej,  it  will  vibrate  ten 
times  as  fast. 

Does  the  rapidity  of  vibration  depend  upon  the 
•weight  of  the  cord  ? — The  wire-wound  string  of  a 
violin  is  much  heavier  than  one  which  is  not  wound, 
and  we  iind  that  it  vibrates  more  slowly.  The  heavier 
the  cord  the  slower  the  vibration.  This  is  always  true. 

If,  to  be  more  particular,  we  take  one  cord  four  times 
as  heavy  as  another,  in  all  things  else  they  beii:g  alike,  it 
will  vibrate  only  one  half  as  fast.  In  all  cases  it  will  be 
true  as  it  is  in  this  one,  that  the  number  of  vibrations  in  a 
second  is  inversely  as  the  square  root  of  the  weight  of  the 
cord.  If,  for  another  example,  we  suppose  one  cord  to 
weigh  16  times  as  much  as  another  of  the  same  length,  it 
will  vibrate  only  \  as  fast. 

What  is  the  third  thing  on  which  the  rapidity 
of  vibration  depends  ? — The  rapidity  of  vibration  de- 
pends also  on  the  weight  or  force  by  which  the  string  is 
stretched.  This  weight  or  force  is  called  the  tension  of 
the  cord.  If,  for  example,  the  weight  which  stretches  the 
cord  over  the  bridges  in  Fig.  27  is  56  Ibs.  the  tension  of 
the  cord  is  said  to  be  56  Ibs. 

Xow  we  find  that,  when  other  things  are  equal,  the  cord 
will  vibrate  faster  as  the  tension  is  made  greater.  If  the 
tensions  of  three  cords  are  as  the  numbers  1,  4,  9,  the 
number  of  vibrations  a  second  will  be  as  1.  2,  3.  But  these 
last  numbers  are  the  square  roots  of  the  first,  in  their 
order,  and  this  teaches  us  that  the  number  of  vibrations  a 
tsecond  is  directly  as  the  square  root  of  the  tension  of  th<i 
cord. 


VIBRATIONS. 


73 


To  what  are  these  principles  applied?— In  the 

manufacture  of  musical  instruments  in  which  the  sounds 
are  made  by  vibrating  cords  or  wires,  such  as  the  guitar 
and  the  piano,  these  principles  become  important.  The 
cords  must  each  be  of  the  right  length  and  weight  and 
tension,  or  else  they  will  fail  to  give  the  correct  tones  for 
music. 

Describe  the  vibration  of  a  wire  fixed  at  one 
end. — When  a  wire,  or  even  a  slender  rod  of  wood,  is 
fastened  at  one  end  in  a  vise,  and  its  other  end  is  drawn 

Fig. 


74  NATURAL  PHILOSOPHY. 

aside,  it  will  spring  back  and  forth  so  swiftly  as  to  appear 
to  be  flattened  out  into  a  gauze-like  fan  (Fig.  28). 

On  -what  does  the  rapidity  of  its  vibration  de- 
pend ? — If  the  rod  is  shortened  it  will  vibrate  faster ;  in 
a  word,  the  number  of  vibrations  in  a  second  will  depend 
on  the  length  of  the  wire  or  rod.  The  law  seems  to  be 
different  from  that  which  applies  to  cord?.  It  states  that 
the  number  of  vibrations  in  a  second  is  inversely  as  tlte. 
square  of  the  length  of  the  rod.  According  to  this  law, 
if  one  rod  36  inches  long  makes  1  vibration  a  second, 

Fig.  29. 


mother  one  only  12  inches  long  will   make  9  in  the  same 
time ;  being  ^  as  long  it  vibrates  9  times  as  fast. 


VIBRATIONS.  Y5 

How  may  the  vibrations  .of  a  bell  be  shown  ? — 
In  Fig.  ^9  we  are  shown  a  hell—haped  glass  vessel  with  a 
little  pendulum-ball  hanging  beside  it.  By  drawing  a 
violin- bow  across  the  edge  of  this  bell  we  make  the  glass 
vibrate,  and  we  shall  know  that  the  vibrations  are  made 
because  the  little  prndulum-ball  will  fly  back  and  forth 
with  a  violent  clatter.  The  edge  of  the  glass,  springing 
back  and  forth,  puts  the  ball  in  motion. 

This  is  one  of  the  many  cases  of  vibration  in  which  the 
motion  is  too  delicate  to  be  seen,  and  the  existence  of 
which  would  not  be  known  if  some  way  had  not  been 
discovered  by  which  to  make  the  vibrations  show  them- 
selves. Cases  of  such  invisible  vibrations  are  very  com- 
mon. In  fact,  almost  every  solid  body  we  can  see  around 

Fig.  30. 


us  is  already  or  else  may  be  put  into  a  state  of  invisible 
vibration. 


76  NATURAL  PHILOSOPHY. 

How  may  the  vibrations  of  a  brass  plate  be 
shown? — If  a  plate  of  brass  is  fastened  at  its  center,  and 
a  violin-bow  be  drawn  across  its  edge  (Fig.  30),  it  will 
vibrate.  Its  vibrations  may  be  felt  by  gently  touching 
it  with  the  finger,  but  by  sprinkling  fine  sand  all  over 
the  plate  its  vibrations  will  be  shown  in  a  curious  and 
much  more  satisfactory  way.  The  picture  represents 
the  beautiful  effect  which  will  be  produced.  The  sand 
will  dance  about,  and  finally  collect  into  straight  lines 
and  curves,  sometimes  in  one  form  and  sometimes  in 
another,  changing  with  every  change  in  the  point  to 
which  the  bow  is  applied  or  upon  which  the  finger  is 
laid.  The  reason  for  this  curious  arrangement  is,  that 
some  parts  of  the  plate  vibrate  more  than  others,  and 
the  sand  gathers  itself  upon  those  points  where  there  is 
the  least  motion. 

How  may  the  vibration  of  water  be  shown  ?— 

Let  the  glass  vessel  (Fig.  29)  be  almost  filled  with  water, 
and  the  bow  then  drawn  across  its  edge.  The  fiuid  will 
be  thrown  into  violent  commotion.  Hosts  of  little  wave- 
lets will  be  thrown  up  and  down  in  quick  succession  upon 
its  surface,  the  water  being  thrown  into  vibration  by  the 
vibration  of  the  glass. 

By  skillfully  drawing  the  bow  these  wavelets  may  be 
brought  into  four  and  sometimes  into  six  beautiful  groups 
separated  from  each  other  by  portions  of  water  which 
seem  to  be  at  rest.  Not  many  effects  so  fine  can  be  so 
easily  produced. 

How  may  water-waves  be  made  on  a  larger 
scale  ? — By  tossing  pebbles  into  the  water  of  a  quiet  lake 
or  pond  we  may  cause  circular  waves  which  spread  away 
farther  and  farther  from  the  place  where  the  pebble  was 


VIBRATIONS.  77 

dropped.  By  watcbing  the  motion  of  the  water  in  these 
waves  we  may  easily  study  the  character  of  such  vibra- 
tions. 

In  what  direction  does  the  water  really  move  ? 
— While  looking  at  the  growing  circles  of  water  started  by 
the  pebble  it  is  most  natural  to  think  that  the  water  is 
moving  just  as  it  appears  to  be — outward  in  all  directions ; 
and  yet  it  is  easy  to  show  that  it  is  not. 

Little  sticks  and  straws  upon  its  surface  would  be  car- 
ried along  outward  too,  if  such  were  the  motion  of  the 
water,  hut  we  tind  that  all  the  sticks  and  straws  will  do 
is  simply  to  rise  and  fall,  which  shows  that  the  real  mo- 
tion of  the  water  is  up  and  down  only,  and  not  outward 
from  the  pebble  as  it  appears  to  be. 

How  is  the  motion  in  the  billows  of  the  sea  ? — 
The  waves  of  the  sea  are  of  the  same  nature.  The  force 
of  the  wind,  however,  drives  the  water  along  at  the  same 
time  that  it  is  vibrating  up  and  down.  But  the  billows 
roll  long  after  the  wind  has  ceased  to  blow,  and  even 
they,  at  such  times,  are  not  able  to  carry  along  the  bits 
of  wood  and  other  light  bodies  that  may  be  floating  upon 
them.  Each  wave  will  seem  to  roll  out  from  under  all 
such  bodies,  and  let  them  down  into  the  furrow  to  be 
lifted  again  by  the  next ;  and  this  shows  that  the  real 
motion  of  the  water  is  simply  a  motion  up  and  down. 

Does  the  air  vibrate  ? — The  air  is  so  elastic  that  it 
yields  to  every  force,  even  the  very  slightest,  and  then 
afterward  springs  back  again.  On  this  account  it  is  in  a 
state  of  vibration  all  the  time.  We  can  not  stir  a  hand 
without  causing  the  air  to  vibrate.  It  is  made  to  tremble 
by  every  breath  we  exhale,  and  it  quivers  at  every  motion 
of  our  lips. 


SOUND. 


How  is  the  sound  of  a  piano-wire  produced  ?— If 

we  look  careliilly  at  a  piano-wire  while  it  is  giving  its 
sound  we  can  often  see  that  it  is  in  motion  ;  its  appear- 
ance will  be  much  like  that  shown  in  Fig.  27.  And  even 
if  its  delicate  motions  can  not  be  seen,  they  may  often  be 
felt  by  placing  the  finger  very  gently  upon  the  wire,  or 
heard  as  a  violent  clatter  if  a  knife-blade  be  used  instead 
of  the  finger.  The  fact  is  that  the  sound  of  the  wire  is 
produced  by  these  rapid  vibrations. 

Are  other  sounds  produced  in  the  same  -way  ? — 
By  laying  the  finger  gently  upon  the  head  of  a  drum  a 
tremulous  motion  will  be  felt  whenever  the  sound  of  the 
drum  is  heard.  And  in  Fig.  29,  while  the  motion  of  the 
little  ball  shows  that  the  glass  beside  it  is  vibrating,  a 
ringing  sound  is  at  the  same  time  heard,  and  whenever  a 
bell  of  any  kind  is  ringing  it  may  be  shown  that  it  is  also 
vibrating.  A  body  always  vibrates  when  it  emits  a 
sound ;  all  sounds  are  produced  by  vibrations. 

Do  all  vibrations  produce  sound? — When  a  cord  is 
long  and  not  too  tightly  stretched  (Fig.  27),  its  vibrations 
may  be  easily  seen,  but  no  sound  will  be  heard. 

A  sudden  motion  of  the  hand  puts  the  air  in  vibration, 
but  gives  no  sound. 

These  and  many  other  illustrations  which  might  be 
given  teach  us  that  all  vibrations  do  not  produce  sound. 


SOUND.  79 

What  is  the  reason  that  some  vibrations  do  not 
produce  sound  ? — If  we  make  the  string,  Fig.  27,  vi- 
brate faster  and  faster,  by  increasing  the  weight  which 
stretches  it,  we  shall  very  soon  be  able  to  hear  a  sound 
which  it  gives,  and  then  if  the  weight  be  made  less  again 
the  sound  becomes  inaudible.  This  shows  that  when 
the  sound  is  not  heard  the  vibrations  of  the  cord  are  too 
slow  to  produce  it.  Some  vibrations  do  not  produce 
sound  because  they  are  too  slow.  The  slowest  vibrations 
that  may  produce  sound  are  at  the  rate  of  16  a  second. 

Is  there  another  reason  why  vibrations  do  not 
produce  sound  ? — A  string  or  wire  may  be  made  so  short 
and  tense  that  its  vibrations  can  not  be  heard.  It  then 
vibrates  so  very  fast  that  no  sound  is  made  by  its  motion. 
Some  vibrations  are  too  fast  to  produce  sound.  The  most 
rapid  vibrations  that  can  be  heard  are  made  at  the  rate  of 
38,000  a  second. 

What  are  sound- vibrations  ? — All  vibrations  be 
tweeii  the  limits  of  16  a  second  and  83,000  a  second 
may  be  called  sound-vibrations.  No  others  ought  to 
be  so  called  because,  if  any  are  either  slower  than  16  a 
second  or  swifter  than  38,000  a  second  they  can  not  be 
heard. 

Is  this  true  for  all  persons? — There  are  persons, 
however,  to  whose  ears  these  limits  will  not  apply.  Some 
ears  are  more  sensitive  than  others ;  they  can  hear  sounds 
that  others  can  not.  The  squeak  of  a  bat  is  made  by  very 
rapid  vibrations,  and  it  is  said  that  there  are  persons  who 
can  not  hear  it  at  all.  The  same  thing  may  be  said  of  the 
chirping  of  a  cricket ;  two  persons  sitting  in  the  presence  of 
one  of  these  animals  may  have  ears  of  such  different  power 
that  while  one  is  annoyed  by  the  shrillness  of  the  cricket's 
voice,  the  other  may  not  know  that  there  is  any  noise 


80  XATCJRAL  PHILOSOPHY. 

produced.      The   limits  named   must,  therefore,  not  be 
taken  as  absolute. 

Will  sound- vibrations  pass  through  air  ? — We  could 
not  hear  the  sound  of  a  distant  bell  if  the  air  did  not  bring 
its  vibrations  to  our  ears.  But  we  may  describe  more 
carefully  the  way  in  which  the  sound  comes  to  us.  In 
the  first  place,  the  ringing  bell  is  itself  vibrating,  as  we 
know,  and  by  these  vibrations  the  air  which  is  in  contact 
with  the  metal  is  made  to  vibrate  also.  This  air  causes 
another  portion  beyond  it  to  vibrate,  and  that  in  turn 
gives  motion  to  another  portion  still  farther  away,  until 
finally  the  air  in  contact  with  the  ear  is  made  to  vibrate 
and  then  we  hear  the  sound.  Yery  quickly  after  the 
hammer  strikes  the  bell,  all  the  air  between  it  and  the  ear 
is  put  in  motion,  but  not  until  this  motion  has  traveled, 
step  by  step,  from  the  bell  to  the  ear,  can  the  sound  be 
heard. 

In  this  way  all  sounds  made  in  the  air  travel  from 
place  to  place.  In  speaking,  our  lips  intrust  their  mes- 
sages to  the  air  and  the  air  carries  them  to  the  ears  of 
our  friend  and  delivers  them  with  the  most  perfect  accu- 
racy. 

Will  sound-vibrations  pass  through  -water  ? — Let 
two  stones  be  struck  together  under  water  and  the  sound 
of  the  blow  will  be  distinctly  heard  above  the  surface. 
Or  if  the  head  is  plunged  beneath  the  surface  while  the 
blow  is  struck,  the  sound  will  be  heard  more  distinctly 
than  in  air;  its  loudness  will  be  likely  to  surprise  the 
hearer.  These  things  show  that  sound-vibrations  pass 
through  water  with  great  facility. 

Will  sound-vibrations  pass  through  solid  bodies  ? 
— To  show  that  sound-vibrations  also  pass  through  wood, 


SOUND.  81 

let  the  ear  be  placed  upon  one  end  of  a  long  table  while 
a  pin  is  drawn  across  the  other  end.  One  who  tries  this 
experiment  for  the  first  time  may  be  astonished  to  hear  a 
sound  so  loud  as  the  scratch  of  the  pin  will  be.  The 
sound-vibrations  in  this  case  travel  through  the  wood  of 
which  the  table  is  made. 

The  power  of  solids  to  convey  sounds  is  also  illustrated 
when  boys  at  the  station,  waiting  for  a  train  of  cars,  put 
their  ears  upon  the  iron  track  to  learn  whether  it  is  com- 
ing near.  The  sound  of  the  train  can  be  heard  much 
farther  through  the  solid  iron  than  through  the  air. 

"  The  American  Indians  understand  that  solid  bodies, 
even  the  earth  itself,  convey  sound  with  great  facility. 
When  these  wild  and  artful  people  suspect  that  enemies 
are  approaching  they  apply  their  ears  close  to  the  ground 
in  order  to  discover  the  noises  made  by  the  footsteps  of 
their  foes  when  too  far  off  for  the  sound  to  be  conveyed 
through  the  air." 

Can  sound- vibrations  pass  through  a  vacuum  ? — 
We  have  seen  that  sounds  can  pass  through  solids,  liquids, 
and  gases,  but  we  must  remember  that  they  can  not  pass 
unless  one  or  another  of  these  forms  of  matter  is  present. 

This  may  be  shown  by  experiment.  A  bell  may  be 
placed  in  the  receiver  of  an  air-pnmp,  and  be  so  fixed 
that  while  there  it  may  be  struck.  Before  the  air  is 
exhausted  its  ringing  may  be  loud  and  clear,  but  long 
before  the  air  is  all  taken  out  it  will  be  almost  too 
feeble  to  be  heard  at  all.  Sound  can  not  pass  through  a 
vacuum. 

What  is  the  velocity  of  sound  in  different  sub- 
stances ? — Sound  travels  faster  in  some  substances  than 
in  others.  By  very  careful  experiments  it  has  been 
found  that  in  air  whose  temperature  is  61?  F,  sound  goes 


82  NATURAL  PHILOSOPHY. 

at  the  rate  of  1,118  feet  a  second.  It  would  travel  faster 
through  warmer  air  and  more  slowly  in  that  which  is 
colder. 

In  water,  sound  travels  faster  than  in  air ;  its  velocity 
is  about  4,714  feet  a  second. 

In  solid  bodies  the  velocity  of  sound  is  still  greater. 
In  pine  wood,  for  example,  it  travels  at  the  rapid  rate  of 
10,900  feet  a  second. 

Do  all  kinds  of  sound  go  with  the  same  velocity  ? 
— In  air  all  sounds  travel  with  the  same  rapidity.  We 
may,  for  example,  while  listening  to  the  music  of  a  dis- 
tant band,  notice  that  the  heavy  sound  of  the  drum  and 
the  shrill  notes  of  the  fife,  made  at  the  same  moment, 
reach  the  ear  together,  and  this  shows  that  they  must 
come  with  equal  swiftness. 

The  same  thing  is  true  of  all  sounds  whatever.  Loud 
sounds  and  low  sounds,  the  sweetest  and  the  harshest 
alike,  travel  through  the  air  at  the  rate  of  1,118  feet  a 
second.  The  overpowering  roar  of  the  thunder  can  not 
outrun  the  delicate  song  of  the  sparrow.  In  any  medium 
all  sounds  travel  with  equal  velocity. 

How  can  we  measure  distances  by  sound  ? — If 
sound  goes  through  air  at  the  rate  of  1,118  feet  each 
second,  it  will  go  twice  that  distance  in  two  seconds  and 
ten  times  that  distance  in  ten  seconds ;  so  that  we  need 
only  to  multiply  1,118  feet  by  the  number  of  seconds  to 
tell  how  far  the  sound  has  come. 

A  man  is  chopping  wood  in  the  distance,  and  3-011  wish 
to  know  how  far  he  is  away  :  watch  the  motions  of  his  axe 
and  listen  to  the  sound  of  the  blows.  You  will  hear  the 
sound  while  the  axe  is  over  his  head.  The  sound  is  made 
when  the  blow  is  struck,-  and  it  takes  as  long  for  the 
sound  to  come  to  you  as  it  does  for  the  axe  to  be  lifted. 


SOUND.  &v 

If  it  is  one  second  between  the  time  you  see  the  blow  and 
the  time  you  hear  the  sound,  then  the  woodman  must  be 
1,118  feet  away. 

Give  another  example. — In  a  similar  way  you  may 
tell  how  far  oft'  a  flash  of  lightning  occurs.  The  sound 
of  thunder  is  made  when  the  lightning  is  seen,  but  it  is  not 
often  heard  until  several  moments  afterward.  If  you  can 
count  the  seconds  between  the  flash  and  the  roar  you  may 
multiply  1,118  feet  by  the  number,  and  this  will  give  the 
number  of  feet  to  the  place  where  the  lightning  occurred. 

Upon  what  does  the  loudness  of  sound  depend  ? — 

The  harder  one  strikes  a  piano-key  the  louder  the  sound 
of  the  note  will  be,  but  all  that  the  heavier  blow  does  is 
to  make  the  piano-wire  vibrate  through  greater  distances. 
In  all  cases  the  loudest  sounds  are  caused  by  vibrations 
through  the  greatest  distances. 

Now  the  distance  through  which  the  vibrating  particles 
swing  back  and  forth  is  called  the  amplitude  of  vibration, 
and  hence  we  may  say  that  the  louduess  of  sound  depends 
upon  the  amplitude  of  the  vibrations  which  produce  it. 

The  harder  one  strikes  a  bell  the  louder  it  rings,  because 
the  heavier  blow  causes  the  particles  of  the  bell  to  vibrate 
through  greater  distances,  or,  in  other  words,  to  have  a 
greater  amplitude  of  vibration. 

What  effect  does  distance  have  upon  the  loud- 
ness  of  sound  ? — The  deafening  roar  of  a  cataract  when 
heard  through  a  distance  of  several  miles  becomes  a  gentle 
murmur,  and  the  deep-toned  thunder,  if  made  too  far 
away,  can  not  be  heard  at  all. 

A  powerful  human  voice  may  be  heard  at  a  distance  of 
about  700  feet ;  at  greater  than  that  distance  it  is  gener- 
ally too  feeble  to  be  heard. 


84  NATURAL   PHILOSOPHY. 

Sounds,  however  loud  when  near  at  hand,  die  away  in 
the  distance  until  they  become  inaudible. 

"Why  is  this  ? — This  is  because  the  amplitude  of  the 
vibrations  of  the  air  grows  less  and  less  as  the  distance  is 
greater  and  greater,  until  at  last  it  is  too  little  to  affect 
the  ear. 

How  can  we  illustrate  this?— When  a  pebble  is 
dropped  into  quiet  water,  the  height  of  the  circling  waves 
is  greatest  near  the  center  where  the  pebble  is  dropped, 
and  grows  less  and  less  as  they  spread  outward,  until  at 
last  we  can  not  see  the  waves  at  all.  So  when  a  bell  is 
struck,  the  amplitude  of  the  air-waves  near  to  the  bell  is 
greatest,  and  it  grows  less  and  less  as  the  waves  spread 
outward  in  all  directions  from  the  bell,  until  at  last  it  is 
so  little  that  the  sound  can  not  be  heard. 

Which  will  convey  sound  farthest,  solids,  liquids, 
or  gases? — In  solid  bodies  the  vibrations  diminish  slowly, 
and  on  this  account  sounds  may  be  heard  through  them 
at  long  distances.  In  liquids  the  vibrations  diminish  more 
rapidly  than  in  solids,  so  that  through  them  sounds  can 
not  be  heard  as  far.  In  gases  the  vibrations  die  out  still 
faster,  and  hence  sounds  can  not  be  heard  as  far  in  them 
as  in  either  of  the  other  forms  of  matter. 

We  now  see  why  the  sound  of  a  coming  train  of  cars 
can  be  heard  sooner  by  placing  the  ear  to  the  iron  track. 
The  solid  iron  conveys  the  sound  to  the  station,  but  in  the 
air  the  vibrations  die  away  before  they  get  there. 

Can  sound  be  heard  farther  in  air  at  some  times 
than  at  others  ? — When  the  air  is  dense,  as  in  a  cold  still 
winter  morning,  or  damp,  as  it  often  is  in  a  still  June 
evening,  we  all  know  how  much  more  distinctly  distant 
sounds  can  be  heard  than  at  other  times.  The  rattling  of 
carriages,  the  laughing  of  children,  the  songs  of  birdc,  and 


SOUND.  85 

a  thousand  other  sounds,  many  of  which  at  other  times 
would  not  be  noticed,  are  heard  with  surprising  clearness 
in  still  damp  summer  evenings. 

What  is  the  law  ?— But  whether  a  sound  can  be  heard 
through  a  distance  great  or  small,  the  rapidity  with  which 
it  dies  away  is  governed  by  the  following  law : 

The  intensity  or  loudness  of  sound  is  inversely  as  the 
square  of  the  distance  from  its  source. 

We  understand  by  this  law  that  at  twice  the  distance 
sound  will  be  only  one  fourth  as  loud ;  or  at  oue  half  the 
distance  it  will  be  four  times  as  loud. 

Will  a  tube  convey  sound  ? — If  we  put  our  lips  to 
one  end  of  a  long  tube  of  metal,  or  even  of  pasteboard, 
while  a  friend  puts  his  ear  at  the  other  end,  we  tind  that 
even  a  gentle  whisper  will  be  very  distinctly  heard. 

A  gentleman  in  the  city  of  Paris  tried  the  experiment 
with  one  of  the  water-pipes  of  that  city,  and  found  that  a 
word  spoken  in  a  very  low  tone  was  heard  by  a  friend 
three  fourths  of  a  mile  away,  while,  as  we  have  already 
learned,  a  powerful  voice  in  the  free  air  would  not  be 
heard  more  than  one  fifth  of  that  distance. 

Why  can  sounds  be  heard  farther  in  tubes  ?— The 
reason  that  sounds  can  be  heard  so  much  farther  in  tubes 
than  in  free  air  is  that  the  tube  keeps  the  vibrations  from 
spreading  in  all  directions  outward  as  they  do  in  the 
atmosphere.  They  are  all  collected  in  the  tube  and  sent 
forward  together,  and  they  will  not  die  away  as  fast  as 
when  allowed  to  scatter  in  all  directions  in  the  air. 

What  are  such  tubes  called? — Such  tubes  are  called 
speaking  tubes.  They  are  often  placed  in  dwellings  so  that 
the  mistress  may  talk  with  her  servants  even  while  they 
are  in  different  and  distant  rooms.  They  are  also  placed 


86  NATURAL  PHILOSOPHY. 

in  large  hotels  for  a  similar  purpose,  and  in  large  manu- 
facturing establishments  they  are  very  common  and  a 
great  convenience. 

What  is  an  echo? — "Who,  after  loudly  tittering  some 
word  or  sentence,  lias  not  at  some  time  heard  the  sound  of 
his  voice  coming  back  to  him  from  a  distant  wood,  or 
perhaps  from  the  wall  of  a  distant  building?  A  part  of 
the  last  word  spoken,  if  not  the  whole  of  it,  and  sometimes 
more  words  than  the  last  one,  may  be  heard  as  distinctly 
as  if  some  mimic  were  shouting  back  what  he  had  heard. 
Such  sounds  are  called  echoes. 

How  is  an  echo  produced  ? — When  we  throw  a  ball 
perpendicularly  against  the  floor  it  bounds  directly  back 
to  the  hand.  In  the  same  way  when  the  sound  of  our 
voice  strikes  perpendicularly  against  a  distant  wall,  it 
bounds  back  to  us  and  we  catch  it  in  our  ears,  and  thus 
hear  the  word  which  we  uttered,  as  an  echo. 

Or,  if  we  throw  a  ball  obliquely  against  the  floor  it 
bounds  away  obliquely  from  us:  we  can  not  catch  it,  but 
it  may  be  caught  by  another  person  standing  in  the  right 
place.  In  much  the  same  way  if  a  sound  strikes  obliquely 
against  a  distant  wall  it  will  be  thrown  obliquely  away 
from  it,  and  a  person  standing  in  its  path  will  hear  the 
sound  as  if  it  came  to  him  obliquely  from  the  wall.  It 
is  in  this  way  that  we  hear  the  echoes  of  sounds  which  are 
not  made  by  ourselves. 

What  is  reflection  of  sound?  —  When  sound  is 
thrown  back  from  a  surface  against  which  it  strikes,  it  is 
said  to  be  reflected* 

Mention  some  remarkable  echoes.— An  echo  in 
Woodstock  Park  repeats  seventeen  syllables  distinctly. 
Another  near  Milan  repeats  a  single  syllable  thirty  times: 


SOUND.  87 

in  this  case  the  sound  is  thrown  from  one  reflecting  sur- 
face to  another  many  times,  and  each  reflection  sends  an 
eel  10  to  the  hearer. 

"  Placing  himself  close  to  the  upper  part  of  the  wall  of 
the  London  Coliseum,  a  circular  building  130  feet  in 
diameter,  Mr.  Wheatstone  found  a  word  pronounced  to 
be  repeated  a  great  many  times.  A  single  exclamation 
sounded  like  a  peal  of  laughter,  while  the  tearing  of  a 
piece  of  paper  was  like  the  patter  of  hail. " 

MUSICAL    SOUNDS. 

What  -was  Galileo's  experiment  ?— Galileo  passed 
the  back  of  his  knife-blade  quickly  over  the  rough  edge  of 
a  coin  and  produced  a  musical  sound.  You  may  repeat  his 
experiment  easily,  or  in  the  absence  of  a  coin  with  ridges 
upon  its  edge,  you  may  use  a  piece  of  one  of  the  heavier 
strings  of  a  violin  or  piano,  which  are  wound  with  fine 
wire;  or,  indeed,  if  there  is  nothing  better  at  hand,  the 
edge  of  a  small  file  may  be  used.  If  you  pass  the  blade 
slowly  over  one  of  these  rough  surfaces  a  series  of  unpleas- 
ant taps  will  be  heard,  but  puss  it  swiftly  and  you  will 
produce  a  shrill  musical  sound. 

How  may  a  note  be  produced  by  a  slate-pencil  ? 
— ''  Every  schoolboy  knows  how  to  produce  a  note  with 
his  slate-pencil.  Holding  it  vertically  and  somewhat 
loosely  between  the  fingers,  on  moving  it  over  the  slate 
a  succession  of  taps  is  heard.  By  pressure  these  taps  can 
be  caused  to  follow  each  other  so  quickly  as  to  produce  a 
continuous  sound.  We  must  not  call  it  musical,  because 
this  term  is  associated  with  pleasure,  and  the  sound  of  the 
pencil  is  not  pleasant."  But  it  is  a  continuous  tone,  and 
yet  we  know  that  it  is  made  up  of  separate  taps  of  the 
pencil. 


NATURAL  PHILOSOPHY 


Fig.  81. 


May  other  separate  sounds  be  made  to  cause  a 
musical  sound  ? — Any  sound  whatever,  repeated  fast 
enough,  will  produce  a  continuous  sound  or  a  tone.  To 
illustrate  this  curious  fact  still  further,  suppose  a  stiff  card 
is  held  against  the  edge  of  a  cog-wheel  turning  slowly ; 
the  card  will  strike  each  cog  that  comes  around  and  every 
blow- will  be  heard  separately,  but  when  the  wheel  turns 
swiftly  the  same  blows,  made  faster,  produce  a  continuous 
and  shrill  musical  sound. 

If  a  watch  could  tick  fast  enough,  say  80  or  100  times 
a  second,  no  one  would  be  able  to  count  the  ticks,  or  even 
to  hear  them  separately,  for  they  would  produce  a  steady 
musical  sound. 

Could  the  blows  of  a  hammer  upon  an  anvil  be  made  fast 
enough  and  with  regularity,  they  would  produce  a  clear 
and  powerful  musical  tone. 

Even  puffs  of  air  when  made 

,^.-  --  y..^^^        *n    quick    succession    result    in 
music.     This  interesting  fact  is 

wll^V      (  ^<l;l'°bt    8nown  by  means  of  the  siren — a 
I    \__^/  Wjx  very  ingenious  instrument  which, 

^Jii .......;. .J'W        we  must  now  describe. 

Describe  the  siren. — In  Fig. 
31  this  instrument  is  represented. 
A  pipe,  P,  enters  a  wind-chest, 
W.  The  top  of  this  chest  is 
pierced  with  a  circular  row  of 
holes.  A  disk  of  metal  lies  very 
near  to  the  top  of  the  wind-chest 
and  is  pierced  with  holes  exact- 
ly corresponding  with  those  in 
the  chest  itself.  The  disk  is  so 
fixed  that  it  may  be  made  to  revolve  very  swiftly,  and 


SOUND.  89 

the  pointers,  like  the  hands  of  a  clock,  seen  at  the  top  of 
the  picture,  are  to  tell  the  number  of  turns  the  disk  makes 
in  a  second. 

Now  suppose  the  holes  in  the  disk  are  exactly  over 
those  in  the  top  of  the  chest.  Then  if  a  blast  of  air  is 
forced  through  the  pipe,  P,  into  the  chest,  it  can  escape 
through  the  holes  in  steady  streams ;  but  let  the  disk  be 
turned  just  a  little  so  that  its  holes  are  not  directly  over 
the  others :  by  this  means  the  holes  in  the  chest  will  be 
closed  so  that  the  streams  of  air  can  not  escape.  As  the 
disk  revolves  the  holes  in  the  chest  will  be  rapidly  opened 
and  shut,  and  the  streams  of  air  will,  in  this  way,  be  cut 
up  into  puffs. 

When  the  disk  turns  slowly  the  separate  puffs  may  be 
heard,  but  when  it  goes  rapidly  these  same  puffs  produce 
a  musical  tone  of  great  purity. 

What  is  necessary,  then,  to  produce  a  musical 
sound  ? — All  that  seems  necessary  to  produce  a  musical 
tone  out  of  any  sound  whatever  is  that  the  separate  pulses 
be  made  with  sufficient  regularity  and  swiftness. 

What  is  meant  by  the  pitch  of  musical  sounds  ? — 

To  distinguish  musical  sounds  as  being  high  or  low,  the 
term  pitch  is  used. 

Some  sounds,  like  the  gravest  notes  of  an  organ, 
are  so  very  low  that  we  can  scarcely  hear  them,  while 
others,  like  the  highest  notes  of  the  piano,  are  so  very- 
high  that  they  are  very  far  above  the  reach  of  any  human 
voice.  Between  the  highest  and  the  lowest  there  may  be 
a  multitude  of  musical  notes  whose  only  difference  is  a 
difference  in  pitch. 

Upon  -what  does  pitch  depend  ?— If  we  take  several 
strings  all  alike,  except  in  length,  we  shall  find  that  the 


90  NATURAL  PHILOSOPHY. 

shortest  string  always  gives  the  highest  sound.  And  now 
if  we  remember  that  the  shortest  string  always  vibrates 
the  fastest,  we  see  that  the  highest  tone  is  the  one  that  is 
made  by  the  most  rapid  vibrations. 

This  is  true  in  all  cases.  The  pitch  of  musical  sound 
depends  entirely  upon  the  rapidity  of  vibration. 

Upon  what  does  the  rate  of  vibration  of  a  string 
depend? — The  rapidity  of  vibration  of  a  string  or  wire, 
as  we  learned  while  studying  the  subject  (p.  71),  de- 
pends upon  the  length  of  the  string,  its  tension,  and  its 
weiy/tt. 

How  then  may  we  vary  the  pitch  of  the  sounds 
of  strings?— If  we  change  either  the  length  of  the  vibra- 
ting string,  or  its  tension,  or  its  weight,  we  change  the 
pitch  of  the  sound  it  produces. 

To  shorten  the  string  makes  the  tone  higher;  to  increase 
the  tension  also  makes  the  tone  higher ;  but  to  increase 
the  weight  of  the  string  makes  the  tone  lower. 

How  is  ihe  pitch  of  the  sounds  obtained  i  i  the 
piano  ? — In  the  piano  the  sounds  are  made  by  the  vibra- 
tion of  wires.  These  wires  are  of  different  lengths  and 
also  of  different  weights;  the  shortest  and  the  lightest 
giving  the  highest  tone.  Now  one  end  of  each  wire  is 
wound  around  a  screw,  and  by  turning  the  screw  in  one 
direction  the  wire  is  tightened  and  the  pitch  raised,  but 
by  turning  it  the  other  way  the  wire  is  loosened  and  the 
pitch  lowered. 

How  is  the  pitch  obtained  in  a  violin  ?— The  violin 
has  four  strings,  each  having  a  weight  different  from  tho 
others,  the  lightest  string  giving  the  highest  note.  Each 
string  is  wound  upon  a  "  key  "  by  which  it  may  be  tight- 
ened or  slackened,  the  tighter  string  giving  the  higher 
note.  They  are  all  of  the  same  length,  but  the  player 


SOUND.  91 

changes  the  length  of  the  vibrating  part  of  eacli  at  pleas- 
ure by  pressing  a  finger  upon  it  at  any  desired  point. 

By  having  the  right  difference  in  the  weight,  the  ten- 
sion, and  the  length  of  the  strings,  the  correct  pitch  of  the 
notes  is  obtained. 

How  is  the  sound  of  the  organ  produced  ? — In  the 
organ  the  musical  sounds  are  caused  by  vibrations  of  col- 
umns of  air.  A  large  bellows  is  kept  full  of  air ;  the  organ- 
ist, by  pressing  the  keys  of  the  organ,  opens  the  way  from 
this  bellows  into  one  or  more  of  the  many  pipes  of  the 
instrument,  and  jets  of  air  going  into  the  pipes  at  the 
bottom,  make  the  air  throughout  their  whole  lengths 
vibrate,  and  these  vibrations  produce  the  musical  sounds. 

Upon  what  does  the  pitch  of  organ-tones  de- 
pend?— The  pitch  of  the  tone  in  an  organ  depends  on 
the  length  of  the  organ-pipe.  The  shorter  the  pipe  the 
higher  the  pitch  will  be. 

The  lowest  note  used  in  music  is  made  by  an  organ- 
pipe,  open  at  the  top,  whose  length  is  32  feet.  If  the 
pipe  is  closed  at  the  top,  a  note  of  the  same  pitch  will  be 
made  by  a  pipe  16  feet  in  length.  The  highest  notes  are 
made  by  pipes  which  are  only  a  few  inches  long. 


LIGHT. 


What  are  luminous  bodies  ?— Luminous  bodies  are 
those  which  produce  light.  The  sun  is  a  luminous  body 
because  it  shines  by  light  which  itself  produces.  The 
flame  of  a  candle,  and  a  red-hot  iron  ball,  are  also  lumin- 
ous bodies  for  the  same  reason. 

What  are  non-luminous  bodies?  —  Non-luminous 
bodies  are  those  that  do  not  produce  light.  If  they  shine 
at  all  it  is  because  they  first  receive  light  from  some  other 
source  and  then  throw  it  off  again.  A  piece  of  rock  or 
of  wood,  a  flower,  a  cloud, — all  these  are  non-luminous 
bodies.  The  rnoon  is  also  non-luminous,  for  while  it  shines 
with  a  steady  and  bright  light,  yet  it  produces  none  itself. 
What  we  call  the  moonlight  is  light  which  goes  from  the 
sun  to  the  moon  first,  and  is  then  thrown  from  the  moon 
to  us. 

What  are  transparent  and  opaque  bodies  ? — Some 
bodies,  like  glass  and  air,  allow  light  to  pass  freely  through 
them  ;  all  such  are  called  transparent  bodies.  Others, 
like  iron  and  wood,  forbid  the  passage  of  light  through 
them  ;  such  are  called  opaque  bodies. 

Is  any  substance  perfectly  transparent  ? — We  do 
not  mean  that  a  transparent  body  will  allow  all  the  light 
that  falls  upon  it  to  go  through.  The  air,  for  example, 
which  is  one  of  the  most  transparent  of  all  things,  does  not 
let  all  the  sunlight  come  through  it.  One  can  often  look 


LIGHT.  93 

directly  at  the  sun,  rising  in  the  morning  or  setting  in  the 
evening,  without  doing  any  injury  to  the  eye,  but  if  the 
air  did  not  shut  out  a  large  portion  of  the  sun's  light  a 
single  glance  would  blind  him.  Perhaps  no  substance 
exists  which  i&  perfectly  transparent. 

Is  any  substance  perfectly  opaque  ? — On  the  other 
hand,  small  quantities  of  light  will  pass  through  wood 
and  even  through  gold,  for  we  all  know  that  it  is  easy  to 
see  through  a  thin  shaving  of  almost  any  kind  of  woo^l, 
and,  if  a  piece  of  gold-leaf  is  at  hand,  we  may  hold  it  be- 
tween the  eye  and  a  window  and  see  that  light  comes 
through  it :  this  light  is  green. 

What  are  rays,  beams,  and  pencils  of  light  ? — A 
ray  is  a  single  line  of  light.  Several  parallel  rays  together 
form  a  beam  of  light. 

The  ray  of  light  is  quite  too  delicate  a  thing  to  be 
obtained  in  practice ;  the  smallest  line  of  light  which  it 
is  possible  to  use  is  made  of  many  rays  ;  it  is  a  beam. 

A  pencil  of  light  is  a  collection  of  rays  that  are  not 
parallel. 

In  what  direction  does  light  go  from  a  luminous 
body  ? — If  a  lamp  is  suddenly  lighted  in  the  center  of  a 
dark  room,  every  part  of  the  room  will  be  instantly  made 
light.  This  shows  that  the  light  goes  from  the  flame  in 
all  directions.  In  the  same  way  light  is  given  from  a 
glowing  coal,  from  a  red-hot  iron,  from  the  sun,  indeed, 
from  all  luminous  bodies,  in  every  possible  direction  at 
once. 

Does  it  move  in  straight  lines  ? — In  order  to  show 
by  experiment  whether  the  path  of  light  is  straight  or 
crooked  we  need  a  darkened  room.  It  is.  easy  to  darken 
a  room  by  closing  the  shutters,  and  at  the  same  time,  if 


94  NATURAL  PHILOSOPHY. 

need  be,  hanging  shawls  or  blankets  over  the  windows. 
It'  a  small  hole  is  made  in  one  of  the  shutters  to  let  the 
sunlight  through,  the  path  of  the  sunbeam  in  the  room 
may  be  distinctly  seen.  If  the  air  is  sprinkled  with  chalk- 
dust  the  beam  is  peculiarly  bright  and  beautiful,  and  no 
artist  could  possibly  draw  a  line  so  absolutely  straight  as 
it  is  seen  to  be. 

We  often  see  such  beams  coming  through  our  half 
closed  window-shutters  and  streaking  the  dusty  air  of  our 
rooms  with  bars  of  light, — beautiful  illustrations  of  the 
fact  that  light  always  moves  in  straight  lines. 

What  other  illustrations  of  this  fact  ?— When  the 
sun  is  sinking  behind  clouds  in  the  western  sky,  it  often 
presents  an  appearance  which  is  very  well  shown  in  the 
picture  (Fig.  32),  and  which  is  sometimes  described  by 
saying  that  the  sun  "draws  water.''  Is  it  possible  that 
any  one  ever  believed  the  streaks  he  saw  were  really 
streams  of  water  being  pulled  up  by  the  sun  ?  We  know 
now  that  they  are  beams  of  li_>ht  from  the  sun,  which  is 
shining  through  openings  in  the  clouds.  They  are  made 
in  just  the  same  way  as  are  the  beams  of  light  seen  in  our 
rooms  when  the  sun  shines  through  openings  in  the  win- 
dow-shutter. 

How  are  shadows  made?— It  is  upon  the  same 
principle  that  shadows  are  made.  To  illustrate,  just  put 
a  book  at  a  convenient  distance  in  front  of  a  lamp-flame. 
Now  some  of  the  rays  of  light  fall  upon  the  book,  and 
can  not  go  farther,  but  others  just  graze  the  edges  of  the 
book  and  pass  in  slraiyht  lines  onward,  so  that  behind  the 
book  and  reaching  to  the  opposite  wall  is  a  space  from 
which  the  light  is  shut  out.  This  apace  which  is  deprived 
of  light  ia  the  shadow  of  the  book. 

We  are  apt  to  call  the  black  spot  on  the  wall  the  shadow, 


LIGHT. 


95 


but  the  shadow  really  reaches  from  the  book  to  the  wall ; 
it  is  all  the  space  behind  the  book  from  which  the  light  is 
shut  out,  and  the  black  spot  on  the  wall  is  only  one  end 
of  it. 

Fig.  32. 


Of  what  two  parts  is  a  shadow  composed  ?— If 

we  examine  the  end  of  the  true  shadow  of  the  book  as  it 
appears  upon  the  wall,  we  may  notice  that  there  is  a  dark 
middle  part,  and  then  a  border  all  around  this,  which  is 
much  lighter.  One  can  hardly  fail  to  see  these  two  parts 
by  examining  the  shadows  made  by  objects  around  the 
evening  lamp.  There  is  in  every  shadow  a  dark  middle 
part  surrounded  by  another  lighter  portion.  The  dark 
middle  part  is  called  thevmbrd,  and  the  lighter  portion  is 
called  the  penumbra.  These  parts  reach  throughout  the 
whole  length  of  every  shadow. 

How  are  they  formed?— A  very  easy  experiment 
will  explain  how  these  two  parts  of  a  shadow  are  pio- 


96  NATURAL   PHILOSOPHY. 

duced.  Place  two  lamp-flames  on  a  table  near  each  other, 
and  hold  a  lead  pencil  or  a  narrow  strip  of  wood  between 
the  flames  and  a  sheet  of  paper,  at  some  distance  from 
them.  Two  distinct  shadows  will  be  seen,  one  cast  by 
each  light.  ISTow  move  the  pencil  nearer  to  the  paper, 
the  shadows  will  approach  each  other,  until  at  last  they 
overlap  and  form  one,  in  which  the  umbra  and  the  penum- 
bra may  be  seen  with  surprising  clearness.  The  umbra 
gets  no  light  from  either  flame,  but  it  is  easy  to  see  that 
every  part  of  the  penumbra  is  getting  light  from  one  or 
the  other.  On  this  account  the  umbra  is  darker  than  the 
penumbra. 

Is  it  so  with  a  single  flame  ? — Now  in  the  shadow 
cast  when  a  single  flame  is  used,  the  outer  parts  are  getting 
light  from  one  ed<je  or  another  of  the  flame,  while  the  mid- 
dle part  is  getting  no  light  at  all  from  any  portion  of  the 
flame.  For  this  reason  the  umbra  is  darker  than  the 
penumbra. 

What  is  the  velocity  of  light  ?— The  light  from  the 
flash  of  a  gun  in  the  distance  comes  to  us  so  very  quickly 
that  it  is  impossible  to  measure  the  brief  time  it  takes. 
So  swiftly  does  lij.ht  travel,  that,  could  it  move  in  curved 
lines,  it  would  go  around  the  world  more  than  seven  times 
in  a  second  !  Its  rate  of  motion  or  velocity  is  about 
190,000  miles  a  second. 

It  will,  of  course,  be  interesting  to  know  how  a  velocity 
so  very  great  could  be  measured.  It  was  first  done  by 
the  Danish  astronomer,  Komer,  about  two  hundred  years 
ago. 

How  was  the  velocity  of  light  found  ? — This  gen- 
tleman, by  observing  the  eclipse  of  one  of  Jupiter's  satel- 
lites, found  out  how  long  it  takes  light  to  go  across  the 


LIGHT.  97 

orbit  of  the  earth,  and  he  then  divided  that  distance  by 
the  number  of  seconds,  and  so  found  out  how  far  the  light 
could  go  in  one  second. 

Explain  the  operation  more  fully. — The  time  when 
an  eclipse  of  Jupit  r's  moon  is  to  begin  is  exactly  known, 
as  it  would  be  seen  when  the  earth  is  in  that  part  of  its 
orbit  which  is  nearest  to  Jupiter.  Now  if  the  earth  is  in 
the  opposite  part  of  the  orbit,  the  eclipse  will  not  be  seen 
to  begin  at  the  moment  at  which  it  is  predicted,  but  16 
minutes  and  36  seconds,  or  996  seconds  afterward.  It  is 
seen  by  light  which  comes  from  it,  and  the  reason  it  begins 
late  is  that  the  light  has  so  much  farther  to  come.  In  fact 
it  takes  the  light  996  seconds  to  go  across  the  orbit  of  the 
earth.  The  distance  across  is  about  190,000,000  miles, 
and  190  millions  divided  by  996  gives  about  190,000  for 
the  number  of  miles  that  light  can  travel  in  one  second 
of  time. 

How  does  distance  affect  the  intensity  of  light  ? 

— The  nearer  a  body  is  to  the  source  of  light,  the  brighter 
or  more  intense  will  be  the  light  that  falls  upon  it. 

Let  us  study  this  subject  more  closely  by  experiment. 
We  may  take  a  sheet  of  paper,  or,  what  is  a  little  better, 
a  square  of  stiff  card  board,  and  hold  it  halfway  between 
a  lamp  flame  and  the  wall  of  a  room.  If  we  measure  the 
shadow  on  the  wall,  we  find  it  to  be  just  twice  as  long  and 
twice  as  wide  as  the  card-board,  and  hence  its  surface  is 
just  four  times  as  large  as  that  of  the  board. 

Now  if  the  light  that  falls  upon  the  card-board  could  go 
to  the  wall  it  would  cover  the  whole  surface  of  the  shadow, 
and  bo  spread  over  a  surface  four  times  as  large  as  that  on 
Avhich  it  does  fall.  Being  spread  over  four  times  as  much 
surface  it  can  be  only  one  fourth  as  bright. 
5 


93  NATURAL  PHILOSOPHY. 

We  see  from  this  experiment  that  at  2  times  the 
distance  from  the  flame  the  light  is  only  one  quarter  as 
intense.  If  the  distance  is  taken  3  instead  of  2,  the 
intensity  would  be  only  one  ninth.  The  law  is  this: 

The  intensity  of  light  varies  inversely  as  the  square  of 
the  distance  from  its  source. 

The  light  of  the  stars  is  dim,  but  if  any  one  of  these 
glimmering  points  could  be  brought  as  near  to  us  as  the 
sun,  it  would  be  more  dazzling  than  that  body  is  to  us. 
Or,  if  the  sun  could  be  carried  out  as  far  as  the  stars,  it 
would  be  no  brighter  than  they. 

How  can  we  see  objects  ? — A  luminous  body  is  seen 
.by  means  of  light  which  comes  directly  trom  it  into  our 
eyes.  It  is  the  light  which  comes  from  a  red-bot  iron  ball 
which  enables  us  to  see  it. 

We  see  non-luminous  bodies  also  by  means  of  light  which 
comes  from  them:  it  is  impossible  to  see  any  object  that 
does  not  send  light  into  the  eye. 

How  can  this  be  true  of  non-luminous  bodies  ?— 
Non-luminous  bodies  have  no  light  of  their  own  to  send  to 
the  eye,  but  they  receive  light  from  luminous  bodies,  and 
throw  it  off  again  into  the  air.  This  light  which  they 
throw  off  may  enter  our  eyes,  and  by  this  means  the  body 
becomes  visible. 

All  non-luminous  bodies  are  invisible  in  a  very  cloudy 
night,  because  no  light  from  the  sun  or  moon  or  stars  falls 
upon  them,  and  they  can  send  none  to  the  eye.  All 
objects  in  our  room  disappear  instantly  when  the  lamp  is 
extinguished,  because  no  light  then  falls  upon  them,  and 
we  therefore  get  none  from  them. 

What  is  reflected  light?— This  light  which  is  thrown 
off  from  non-luminous  bodies  is  called  r< fleeted  light. 


LIGHT.  99 

What  experiment  can  illustrate  reflection  of 
light? — Let  abeam  of  sunlight  into  a  darkened  room 
through  a  hole  A,  Fig.  33,  in  the  shutter  of  a  window,  and 
make  it  fall  upon  a  looking-glass.  This  beam  will  bound 
off,  or  be  reflected  from  the  glass,  and  go  up  to  the  ceiling, 
where  it  will  form  a  bright  spot  C.  Let  the  air  of  the 
room  be  sprinkled  with  dust,  and  the  beams  of  light  will 

Fig.  33. 


be  shown  as  clearly  as  they  are  represented  in  the  picture, 
which  is  a  very  good  picture  of  the  experiment. 
What  are  important  parts  to  notice  here  ?— It  is 

very  necessary  to  understand  clearly  the  use  of  certain 
terms  which  are  applied  to  certain  things  represented  in 
this  figure.  There  is  first  the  beam  AB,  which  falls  upon 
the  reflecting  surface,  and  is  called  the  incident  beam; 
and  then  the  beam  BC,  which  is  thrown  from  the  reflect- 
ing surface,  and  is  called  the  reflected  beam.  The  point 
B  is  called  the  point  of  incidence. 

And  now,  if  we   suppose  a  perpendicular  BD  to  be 


100  NATURAL  PHILOSOPHY. 

erected,  we  have  two  angles  made,  one,  ABD,  the  angle 
between  the  incident  beam  and  the  perpendicular,  is  the 
angle  of  incidence  ;  the  other,  CBD,  the  angle  between 
the  reflected  beam  and  the  perpendicular,  is  the  angle  of 
reflection. 

What  is  the  law  of  reflection? — ]STow  these  two 
angles  are  exactly  equal  in  this  experiment :  they  are 
always  so.  The  law  of  reflection  states  that  the  angle  of 
reflection  must  be  equal  to  the  angle  of  incidence. 

What  are  mirrors  ?— Some  bodies  reflect  light  much 
better  than  others.  Yery  little  is  reflected  from  the  sur- 
face of  rough  iron,  for  example,  while  a  great  deal  is 
thrown  from  the  surface  of  new  tin.  Those  which  reflect 
light  most  freely  are  called  mirrors. 

A  common  looking-glass  is  the  most  familiar  example 
of  a  mirror,  but  other  forms  are  almost  as  common.  The 
inside  surface  of  a  bright  silver  spoon  is  a  good  mirror, 
and  the  outside  surface  also,  because  from  both  these  sur- 
faces light  is  very  freely  reflected. 

What  are  three  forms  of  mirror  ? — There  are  three 
forms  of  mirrors  which  need  especial  notice : 

First,  the  plane  mirror,  one  whose  surface  is  like  that 
of  the  looking-glass,  plane  or  flat. 

Second,  the  concave  mirror,  one  whose  surface  is  like 
that  of  the  inside  of  the  silver  spoon,  hollowed  or  concave. 

Third,  the  convex  mirror,  one  whose  surface  is  like  the 
outside  of  the  silver  spoon,  rounded  or  convex. 

What  effect  do  plane  mirrors  cause? — If  light 
from  any  object  falls  upon  a  plane  mirror,  and  after  re- 
flection enters  our  eyes,  we  see  an  image  of  the  object 
(Fig.  34).  It  appears  to  be  behind  the  mirror,  and  just  as 
far  behind  it  as  the  object  itself  is  in  front  (Fig,  35). 


LIGHT. 

Fig.  34. 


101 


Give  examples. — Our  own  image  seen  in  a  looking- 
glass  is  the  most  familiar  example  of  this  effect,  but  look- 
ing down  into  the  water  of  a  well  or  of  a  lake  will  show 
the  image  just  as  perfectly.  The  surface  of  quiet  water 
is  indeed  a  very  perfect  plane  mirror,  and  forms  images 
of  all  objects  above  it  with  wonderful  clearness.  How 
beautiful  are  the  pictures  of  the  mansions  and  shrubbery 
along  the  bank  of  a  river,  or  near  the  shores  of  a  lake,  as 
we  see  them  presented  in  the  still  water  between  us  and 
them  1 

How  are  images  formed  ? — The  way  in  which  this 


102  NATURAL  PHILOSOPHY. 

curious  effect  is  produced  may  be  understood  by  studying 
Fig.  35,  which  represents  a  boy  looking  at  the  image  of  a 
candle  in  a  looking-glass. 

Fig.  35. 


Rays  of  light  are  shown  going  from  the  tip  of  the  can- 
dle-flame to  the  mirror,  and  being  reflected  from  it  into 
the  eye  of  the  boy.  These  rays  appear  to  have  come  from 
a  point  behind  the  mirror,  and  this  point  is  the  image  of 
the  tip  of  the  flame  from  which  the  rays  first  started. 

Now  every  point  on  the  whole  candle  will  send  rays  to 
the  mirror  to  be  thrown  back  into  the  same  eye,  and  thus 
form  an  image  of  every  point,  or  in  other  words  an  image 
of  the  candle  itself. 

And  so,  when  a  person  sees  his  own  image  in  a  mirror, 
he  may  think  of  its  being  formed  in  the  same  way.  He 
may  think  of  rays  of  light  going  from  every  point  on  his 
person  to  the  mirror,  and  thence,  being  reflected,  coming 
into  his  eyes,  and  then  he  may  think  of  his  eyes  tracing 
these  rays  right  back,  in  the  direction  from  which  they 
last  came,  to  points  behind  the  mirror;  these  points  make 
up  the  image  which  he  sees. 


LIGHT. 


103 


How  may  more  than  one  image  of  an  object  be 
made  at  once  ?— By  using  two  looking-glasses  several 
images  of  one  object  may  be  seen  at  once. 

By  placing  two  looking-glasses  parallel  to  each  other 
and  near  together,  and  then  putting  the  eye  at  one  end, 
while  a  ball  or  other  object  is  put  between  them  at  the 
other,  a  large  number  of  images  of  the  ball  may  be  seen. 
The  picture,  Tig.  36,  shows  how  the  light  must  be  reflected 


back  and  forth  from  one  mirror  to  the  other  to  make  so 
many  images. 

How  may  just  three  be  formed  ? — By  holding  two 
mirrors  at  right  angles  (Fig.  37),  any  object  placed  be- 
tween them  will  give  three  images. 


104 


NATURAL  PHILOSOPHY. 
Fig.  37. 


Fig.  88. 


How  may  five  be  obtained  ?— But  if  instead  of  bein^, 
at  right  angles  the  mirrors 
are  inclined,  as  in  Fig.  38, 
making  an  angle  of  CO0,  just 
live  images  will  be  formed. 
What»is  the  effect  of 
a  ccncave  mirror  upon  a 
team  of  light  ?— Fig.  39 
shows  the  effect  of  a  con- 
cave mirror  on  parallel  rays. 
The  rajs,  after  reflection, 
will  no  longer  be  parallel ; 
they  will  converge  and  cross 
each  other  at  the  point  F. 

This  point  is  called  the  focvs  of  the  mirror.  Any  point 
where  rays,  after  reflection,  cross  each  other,  is  a  focus  ; 
but  when  the  mirror  is  held  perpendicular  to  the  path 


LIGHT. 


105 
the 


of  the  beam,  as  in   the  picture,  the  focus  is  called 
pr  Inc  tpal  focus. 

What  is  the  effect  of  the  concave  mirror  on  di- 
verging rays  ?— Let  a  candle-ilame  be  held  some  distance 

Fig.  39. 


before  a  concave  mirror  (Fig.  40),  and  the  light  reflected 
will  make  a  bright  spot  upon  the  finger  or  any  thing  else 


Fig.  40. 


held  at  the  point  F.  We  see  that  while  the  rays  from  the 
candle  are  diverging,  those  which  are  thrown  from  the 
mirror  are  converging. 

What  effect  does  a  concave  mirror  always  pro- 
duce ? — In  both  these  pictures  we  notice  that  the  rays, 
after  reflection,  are  brought  nearer  together  than  they 
were  before,  or,  in  other  words,  that  they  are  collected  by 
reflection.  A  concave  mirror  always  makes  the  rays  go 
on  nearer  together  after  reflection  than  they  did  before. 
' 


106  NATURAL  PHILOSOPHY. 

Describe  the  image  formed  by  a  concave  mirror 
•when  an  object  is  placed  very  near. — By  holding 
one  of  these  instruments  very  near  to  the  face,  a  person 
can  hardly  recognize  himself  in  the  immense  image  which 
he  sees.  The  picture  (Fig.  41)  represents  a  concave  mir- 
ror, M,  with  a  young  man  standing  very  near  to  it ;  his 


Fit:.  41. 


face  ii  supposed  to  be  between  the  focns  and  the  mirror. 
The  image  formed  is  much  larger  than  the  object,  and 
seems  to  be  almost  upon  the  surface  of  the  mirror,  instead 
of  being  as  far  behind  it  as  we  are  accustomed  to  see  it  in 
a  common  looking-glass. 

.The  concave  mirror  always  forms  an  image  larger  than 
the  object  if  the  object  is  very  near  to  it. 

Suppose  the  object  i^  moved  away  from  the 
mirror? — If  a  candle  is  put  a  little  farther  away  from 
the  mirror  than  the  focus,  a  still  more  curious  effect  will 
be  produced.  The  image  will  not  appear  behind  the 
mirror  at  all ;  it  wil]  be  in  front  of  it  and  farther  away 
than  the  object  itself,  and,  curiously  enough,  always  bot- 
tom upward,  or  inverted. 

The  picture  (Fig.  42)  represents  the  experiment.     The 


LIGHT.  107 

image  could  not  be  seen  if  a  screen  were  not,  put  in  just 
the  right  place  to  receive  it,  because  no  light  would  come 


Fig.  42. 


from  it  to  our  eyes.     The  screen  receives  the  light  which 
forms  the  image  and  reflects  it  to  the  eye. 

Now  let  the  object  be  moved  still  farther  away. 
— If  the  candle  is  carried  etiH  farther  away  from  the 
mirror,  the  image  will  move  up  toward  it  and  grow  smaller. 
At  length  the  object  and  the  image  will  be  at  equal  dis- 
tances from  the  mirror;  they  will  then  be  of  equal  size, 
but  as  the  candle  is  carried  out  farther  and  farther  the 
image  will  go  nearer  and  nearer  to  the  mirror,  growing 
smaller  and  smaller  all  the  time,  until  finally  it  appears 
only  as  a  bright  spot  at  the  focus. 


108 


NATURAL   PHILOSOPHY. 


The  concave  mirror  is  the  only  form  that  can  produce 
images  in  front  of  it,  or  that  are  inverted. 

Describe  the  image  by  a  convex  mirror. — Fig.  43 
shows  the  small  image  which  i.s  formed  when  a  person 


Fig.  <». 


looks  into  a  convex  mirror.  It  is  always,  as  we  see  it 
here  represented,  behind  the  surface  of  the  mirror,  erect, 
and  smaller  than  the  object. 

i r««f-  44.  How  is  light  reflected 

from  rough  surfaces  ? 

— When  a  beam  of  light 
falls  upon  a  rough  sur- 
face it  is  reflected  irregu- 
larly. This  effect  is  shown 
in  Fig.  4i.  The  result  is 
that  the  reflected  light 
is  scattered  in  every  di- 
rection. 

Do  the  rays  obey  the  law  of  reflection  ? — When 
we  say  that  the  reflection  is  irregular,  we  do  not  mean 
that  the  rays  are  not  thrown  off  according  to  the  law  of 
reflection.  That  law  is  never  transgressed.  Every  ray 


LIGHT.  109 

must  be  thrown  in  such  a  direction  that  the  angle  of  reflec- 
tion is  equal  to  the  angle  of  incidence.  But  on  a  rou«rh 
surface,  like  that  seen  in  Fig.  44,  the  reflecting  points  are 
not  in  regular  order,  and  for  this  reason  the  reflected 
rays  are  not. 

Do  transparent  bodies  reflect  light? — Transpar- 
ent bodies  do  reflect  light :  we  know  this  from  the 
simple  fact  that  we  are  able  to  see  them ;  for  we  have 
learned  before,  that  an  object  is  visible  only  by  the  light 
which  is  thrown  from  it  into  our  eyes.  Glass  and  water 
are  transparent  bodies,  and  yet  these  bodies  are  able  to 
reflect  a  great  deal  of  light. 

How  do  they  differ  from  opaque  bodies  ?— An 
opaque  body  does  not  allow  light  to  pass  through  it ;  a 
transparent  body  allows  the  light  to  pass  through  it 
freely.  For  instance,  we  can  not  look  through  a  piece 
of  sheet-iron  ;  it  stops  the  light  and  reflects  it,  but  we  see 
distinctly  through  glass,  because  it  can  reflect  only  a  part 
of  all  the  light  that  falls  upon  it  and  allows  the  rest  to 
come  through. 

Can  this  be  shown  by  experiment  ? — Water  is  as 
transparent  as  glass,  and  we  can  easily  show  that  water 
does  not  reflect  all  the  light  that  falls  upon  its  surface. 
The  picture  (Fig.  45)  tells  us  at  once  how  such  tin  experi- 
ment is  to  be  made.  A  glass  vessel  iilled  with  water  is 
placed  so  that  a  beam  of  sunlight  coining  into  a  dark  room 
through  a  hole  in  a  shutter  can  fall  upon  the  surface  of 
the  fluid.  The  beam  of  light  can  be  seen  in  its  passage 
through  the  water. 

All  transparent  bodies  allow  light  to  go  through  them 
in  this  way,  but  it  is  not  always  possible  to  see  the  path  of 
the  rays. 


110 


NATURAL  PHILOSOPHY, 
Fig.  45. 


What  else  may  be  noticed  in  the  experiment? — 

If  the  air  in  the  room  is  a  little  dusty,  the  path  of  light 
from  the  window  to  the  water,  as  well  as  through  the 
water  itself,  can  be  seen,  and  then  it  will  be  noticed  that 
the  beam  seems  to  be  broken  just  at  the  point  where  it 
enters  the  liquid.  This  is  also  shown  in  the  picture. 
The  beam  is  perfectly  straight  while  in  the  air,  arid  per- 
fectly straight  while  in  the  water  also,  but  it  is  bent  just 
where  it  passes  from  one  into  the  other. 

Is  this  always  the  case  ? — This  bending  of  the  raya 
of  light  in  passing  from  one  transparent  substance  into 
another  generally  takes  place. 

The  light  that  comes  through  the  windows  into  our 
houses  is  bent  twice,  once  when  it  passes  from  the  air 


LIGHT.  in 

outside  into  the  glass,  and  again  when  it  passes  from  the 
glass  into  the  air  in  the  room. 

What  is  this  bending  of  the  rays  called? — This 
bending'  of  the  rays  of  light  in  passing  from  one  substance 
into  another  is  called  refraction. 

What  terms  must  now  be  understood  ? — To  ex- 
plain certain  terms  used  in  connection  with  the  subject  of 
refraction  we  must  first  suppose  a  line  to  be  drawn  per- 
pendicular to  the  surface  where  refraction  occurs,  and  to 
be  extended  both  ways  from  it,  or  into  both  substances. 
In  Fig.  4fi,  for  instance,  we  must  suppose  a  line  perpen- 
dicular to  the  water  at  the  Fig.  46. 
point  I,  where  the  light  H  I 
enters,  and  to  reach  up  into 
the  air  and  down  into  the 
water.  Then  we  have, 

The  incident  beam — that 
which  falls  upon  the  sub- 
stance into  which  it  is  to 
pass,  R  I: 

The  refracted  learn — that 
which  passes  through  the 
second  substance,  I  S  : 

The  angle  of  incidence— -the  angle  between  the  incident 
beam  and  the  perpendicular : 

The  angle  of  refraction— the  angle  between  the  refract- 
ed beam  and  the  perpendicular. 

On  what  does  the  amount  of  refraction  depend  ? 
— Some  substances  bend  the  rays  more  than  others.  The 
amount  of  bending  depends  upon  the  difference  in  the 
density  of  the  two  substances.  If  two  substances  could 
have  exactly  the  same  density,  light  would  pass  from  one 
to  the  other  without  being  bent  at  all,  but  if  one  is  more 


112  NATURAL  PHILOSOPHY. 

dense  than  the  other,  the  bending  will  occur.  For  in- 
stance, water  is  more  dense  than  air,  so  that  light  which 
passes  either  from  air  into  water  or  from  water  into  air, 
will  be  retracted. 

What  is  the  law  of  refraction  ? — If  the  rays  are 
passing  from  one  medium  into  a  denser  one  the  angle  of 
refraction  will  be  smaller  than  the  angle  of  incidence,  but 
if  passing  into  one  which  is  less  dense  the  angle  of  refrac- 
tion will  be  larger  than  the  angle  of  incidence. 

If,  for  illustration,  the  beam  R  I  (Fig.  46)  is  the  incident 
beam  going  from  air  down  into  water,  it  will  be  bent  so 
as  to  go  in  the  direction  I  S.  Now  it  h  easy  to  see  that 
the  angle  S I  P,  in  the  water,  or  the  angle  of  refraction,  is 
smaller  than  the  angle  in  the  air,  or  the  angle  of  inci- 
dence. 

But  suppose  S  I  represents  a  beam  going  from  the 
water  up  into  the  air  above,  it  will  be  bent  so  as  to  go  in 
the  direction  of  I  R.  In  this  case  the  angle  in  the  air  is 
the  angle  of  refraction :  the  angle  of  incidence  is  in  the 
water.  The  light  is  passing  into  a  less  dense  substance 
and  the  angle  of  refraction  is  larger  than  the  angle  of 
incidence. 

How  does  a  straight  stick  appear  when  partly 
plunged  into  water? — If  a  straight  stick  is  plunged  a 
part  of  its  length  into  clear  water  it  will  look  as  if  it  were 
broken  or  bent  just  at  the  surface  of  the  water.  This 
effect  ij  caused  by  refraction,  and  it  is  very  common. 
Boatmen  arc  especially  familiar  with  it,  since  their  oars 
always  look  bent  at  the  point  where  they  enter  the  water. 
(Fig.  47.) 

Why  does  the  stick  appear  to  be  bent  ? — Of  course 
the  oar  or  stick  is  not  bent,  and  it  looks  to  be  so  only  be- 
cause the  light  that  cornea  wit  of  the  water  from  it  is  bent 


113 


Fig.  48. 


when  it  enters  the  air,  and  the  part  that  is  in  the  water 
will  appear  to  be  in  the  direction  in  which  the  light  from 
it  enters  the  eve.  All  this  may  be  understood  if  you 
carefully  examine  Fig.  4£,  where  the  true  place  of  the 

stick  in  water  is  shown 
b}r  dotted  lines,  and  two 
rays  that  come  from 
the  lower  end  of  it  are 
shown  bent  at  the  sur- 
face of  the  water,  so 
as  to  enter  the  eye, 
which  traces  them  back 
in  straight  lines,  so  that 
they  seem  to  come  from 
a  point  above  the  ere 
from  which  they  started. 


114  NATURAL   PHILOSOPHY. 

What  is  another  curious  effect  of  refraction  ? — 

Water  does  not  appear  to  be  as  deep  as  it  really  is  on 
account  of  the  retraction  of  the  light  which  comes  from 
the  bottom  into  the  air.  The  young  man  represented  in 
Fig.  50  sees  the  bottom  of  the  water  lifted  far  toward  the 
surface.  The  rays  of  light  from  the  point  O,  for  example, 
are  refracted  at  the  surface  rig.  49. 

and  by  this  means  enter 
the  eye  as  if  they  came 
from  O'  ;  and  so  the  bot- 
tom at  0  seems  to  be 
at  O'. 

What    are    lenses  ?— 

The   instruments   used   to 
refract  light  are  called  len- 
ses ;  they  are  transparent  bodies  having  either  one  or  two 
curved  surfaces. 

Fig.  4:9  is  a  picture  of  one  kind  of  bns.  Looking  at 
the  side  of  the  instrument  it  is  circular ;  looking  at  the 
edge  of  it,  it  is  found  to  be  thicker  through  the  middle ; 
both  sides  are  convex.  This  is  called  the  double  convex 
lens. 

Sometimes  both  sides  of  the  lens  are  concave  and  the 
middle  is  thinner  than  the  edge  ;  in  this  case  it  is  called  a 
double  concave  lens. 

How  many  kinds  are  there  ? — There  are  six  differ- 
ent forms  of  lens.  They  are  shown  in  section  by  Fig.  51. 
They  are  named  as  follows  : 

1  The  double  convex.  4  The  double  concave. 

2  "    piano  convex.  5      "    piano  concave. 

3  "    meniscus.  G       "    concavo  convex. 
The  first  three  act  upon  light  in  the  same  way :  on 


115 


tins  account  only  the  first  or  double  convex  lens  will  be 
particularly  noticed. 


The  last  three  are  alike  also  in  their  effect,  and  we  need 
give  close  attention  only  to  the  action  of  the  double 
concave. 

What  is  the  effect  of  the  convex  lens  on  parallel 


HO  NATURAL  PHILOSOPHY. 

rays  ? — If  a  double  convex  lens  is  held  in  the  path  of  a 
sunbeam,  especially  in  a  darkened  room,  the  rays  will  not 
come  out  of  it  parallel :  they  will  be  so  bent  as  to  all 
come  to  one  point,  as  we  are  shown  by  Fig.  52. 

Fig.  52. 


The  point  F  (Fig.  52),  where  the  parallel  rays  are  col- 
lected, is  called  the  principal  focus  of  the  lens. 
What  effect  is  produced  on  diverging  rays  ?— We 

may  suppose  the  rays  to  go  froin  the  focus  to  the  lens : 
these  rays  (F  A,*Fig.  52)  are  diverging  and,  as  the  figure 
represents  them,  they  will  be  parallel  after  going  through 
the  lens. 

But  diverging  rays  are  not  always  made  parallel ;  indeed, 
they  will  not  be  unless  they  start  from  the  focus.  If  they 
start  from  a  point  between  the  focus  and  the  lens  they 

Fig.  58. 


LIGHT. 


117 


will  diverge  after  going  through,  but  the  divergence  will 
l>e  less  than  before.  On  the  other  hand,  if  the  rays  start 
from  a  point  farther  than  the  focus  from  the  lens,  they 
will  be  converging  after  refraction.  This  case  is  beauti- 
fully shown  in  Fig.  53. 

Does  the  concave  lens  have  the  same  effect  ? — 
The  concave  lens  has  exactly  the  opposite  effect.  The 
rays  after  passing  through  a  concave  lens  are  separated 
instead  of  being  collected. 

Fig.  54. 


This  effect  is  well  shown   in   Fig.  54,  which  represents 
a  double  concave  lens  refracting  parallel   rays  of  light. 


Fig.  55. 


They  are  supposed  to  enter  the  lens  on  the  side  F,  parallel 
to  each  other,  but  on  coming  out  on  the  other  side  they 


US  NATURAL  PHILOSOPHY. 

are  diverging.  All  the  concave  lenses  have  the  effect  to 
separate  rays  by  retraction. 

Describe  the  image  formed  by  a  convex  lens. — 
Most  perfect  and  very  beautiful  images  are  formed  by  the 
use  of  convex  lenses.  If  one  of  these  instruments  is  held 
at  a  little  distance  from  any  object,  a  flower,  for  example, 
it  will  form  an  image  which  may  be  caught  upon  a  screen 
placed  in  the  right  spot.  This  image  will  be  on  the  other 
side  of  the  lens  from  the  object,  and  inverted  (Fig.  55). 

Explain  the  production  of  the  image. — Fig.  56 
and  Fig.  57  will  help  us  to  understand  how  this  image  is 

Fig.  56. 


formed.  They  show  a  lens  with  a  small  arrow,  a  J,  near 
to  it.  Two  rays  of  light  are  seen  going  from  a  through 
the  lens,  and  after  refraction  meeting  again  at  A.  This 
point,  A,  is  the  image  of  the  point,  «,  from  which  the  rays 
started.  Two  other  rays  are  seen  going  from  5  through 
the  lens  and  being  refracted  to  the  point  13.  This  point 
B  is  the  image  of  the  other  end  of  the  arrow.  Every 
point  between  a  and  b  in  the  arrow  will  send  off  rays  of 
light  which,  after  going  through  the  lens,  will  be  brought 
together  again  at  corresponding  points  between  A  and  B, 
and  all  together  they  make  up  the  whole  image  A  B. 
When  will  the  image  be  larger  than  the  object  ? 


LIGHT. 


119 


— Tf  the  image  is  farther  than   the  object  from  the  lens 
(see  Fig.  56),  it  will  be  larger  than  the  object. 

When  will  it  be  smaller  ? — But  if  the  image  is  nearer 
to  the  lens  than  the  object  is  (see  Fig.  57),  it  will  be 
smaller. 

Fig.  57. 


Whichever  is  farthest  from  the  lens  will  be  the 
largest. 

Will  the  image  ever  be  on  the  same  side  of  the 
lens  as  the  object? — In  the  cases  thus  far  examined  we 
suppose  the  object  to  be  outside  or  beyond  the  focus  of 
the  lens.  If  the  object  is  put  between  the  lens  and  its 
focus,  the  image  will  be  seen  on  the  same  side  as  the 
object.  It  will  be  erect,  and  very  much  larger  than  the 
object.  In  Fig.  58  tins  case  is  shown.  A  small  insect  is 
between  the  focus  F,  and  the  lens,  and  a  person  looking 

Fig.  5S. 


120  NATURAL  PHILOSOPHY. 

through  the  lens,  ins'ead  of  seeing  the  little  creature  a  5, 
will  behold  its  magnified  image,  A  B. 

What  is  the  effect  of  concave  lenses  ?— Concave 
lenses  have  just  the  opposite  effect ;  they  form  an  image 
always  smaller  than  the  object.  In  Fig.  5i)  we  can  see 

Fig.  59. 


how  this  is  done.  The  light  from  the  vase  A  B,  after 
going  through  the  concave  lens,  seems,  to  the  eye,  to  have 
come  from  the  smaller  image  a  I. 

Is  all  light  of  the  same  color  ?— The  light  which  the 
sun  sheds  upon  all  things  so  freely,  is  said  to  be  white 
light,  but  yet  all  light  is  not  white.  The  light  of  some 
stars,  for  example,  is  as  red  as  a  flame  of  fire,  while 
others  shed  upon  us  a  delicate  light  as  green  as  that  of  an 
emerald. 

Why  are  bodies  of  different  colors  ? — We  can  see 
an  object  only  by  means  of  the  light  that  ia  reflected  by 
it.  Now,  if  the  light  which  it  reflects  is  red,  then  the 
color  of  the  body  is  itself  red.  If  a  body  reflects  blue 
light,  the  body  ha-  a  bine  color;  in  every  case  the  color 
of  a  body  is  the  color  of  the  light  which  that  body  reflects. 


LIGHT.  121 

The  meadows  are  green  because  the  vegetation  throws 
green  light  to  our  eyes. 

What  curious  experiment  -will  illustrate  this  ? — 

"  Fill  a  spirit-lamp  with  alcohol  in  which  a  large  quantity 
of  salt  has  been  dissolved  ;  on  being  lit  it  will  be  found  to 
burn  with  a  livid  yellow  flame."  Let  a  room  be  lighted 
entirely  by  one  or  two  of  such  lamps.  "It  should,  if  pos- 
sible, be  hung  with  pictures  in  water  and  oil  colors,  and 
the  persons  present  ought  to  wear  nothing  but  the  bright- 
est colors,  and  the  table  be  ornamented  with  the  gayest 
of  flowers."  Let  the  lamps  be  brought  into  this  darkened 
room,  and  an  astonishing  appearance  will  be  presented. 
"  The  furniture  and  every  other  object  which  the  room 
contains  will  reflect  but  a  single  color.  The  brightest 
purple,  the  purest  lilac,  the  liveliest  green  will  be  con- 
verted into  a  monotonous  yellow.  The  same  change  will 
take  place  in  the  countenances  of  those  present:  every  one 
will  laugh  at  the  appearance  of  his  neighbor's  face  without 
thinking  that  he  is  just  as  great  a  subject  of  laughter  to 
them." 

Nothing  can,  better  than  this  experiment,  show  that 
bodies  will  seem  to  be  of  the  color  which  they  can  reflect. 
When  they  receive  only  yellow  rays,  they  can  themselves 
be  of  no  other  color.  And  if  any  of  them  are  not  able  to 
reflect  yellow  light,  these  will  appear  black. 

Then  why,  in  the  sunlight,  are  not  all  bodies 
white  ?— All  bodies  in  the  sunlight  are  receiving  only 
white  light,  and  if  white  light  was  like  that  of  any  other 
color  they  would  all  be  white.  The  white  light  must 
contain  all  other  colors  which  bodies  reflect.  These  bodies 
receive  all  these  color3  alike,  and  then  each  one  makes 
choice  of  the  color  which  it  will  reflect.  A  rose  gets  white 
light  from  the  sun,  and  then  from  among  all  the  colors  it 
6  - 


122  NATURAL  PHILOSOPHY. 

contains,  it  reflects  the  red  only.  A  violet  reflects  blue 
instead  of  red  or  any  other,  while  the  leaves  of  a  tree  re- 
flect only  the  green  rays  of  the  white  light  which  the  sun 
sheds  upon  them. 

By  -what  instrument  can  sunlight  be  separated 
into  its  colors? — The  instrument  used  in  the  arts  to 
decompose  light  is  called  a.  prism.  It  is  generally  nothing 

Fig.  60. 


more  than  a  triangular  piece  of  glass,  but  it  may  be  made 
of  many  other  substances.  Fig.  60  represents  the  prism, 
and  Fig.  61  shows  how  this  instrument  is  often  mounted 
upon  a  stand  to  be  convenient  for  use. 

Describe  the  experiment  with  the  prism.— Let  a 
prism  be  held  in  a  beam  of  sunlight  as  it  enters  a  dark- 
ened room ;  the  rays  which  come  through  the  prism  will 
strike  the  wall  or  ceiling  of  the  room,  or  upon  a  screen, 
and  form  there  a  patch  of  beautifully  colored  light  (Kig. 
62).  All  the  colors  of  the  rainbow  will  be  seen ;  and 
what  is  still  more  beautiful,  if  dust  be  sprinkled  into  the 
air  of  the  room,  these  colors  will  be  seen  reaching  all  the 


LIGHT. 


123 


way  from  the  prism  to  the  wall.     Rays  of  purest  bine,  of 
most  delicate  violet,  of  the  brightest  yellow,  with  others 


Fig.  61. 


of  different  colors,  will  be  seen  spread  out  like  a  fan  from 
the  prism  through  the  dusty  air 

This  arrangement  of  colors  formed  from  the  sunlight 
which  passes  through  a  prism  is  called  the  solar  spectrum. 

What  are  the  colors  in  the  solar  spectrum? — 
There  are  seven  colors  in  the  solar  spectrum.  They  are 
arranged  in  the  following  order :  red,  orange,  yellow, 
green,  blue,  indigo,  and  violet. 

These  are  the  colors  of  which  sunlight  is  composed,  and 
the  colors  of  all  bodies  in  the  world  are  produced  by  the 
mixture  of  two  or  more  of  these  in  different  proportions. 

Will  the  seven  colors  produce  white  light  ?— If 


'124  NATURAL  PHILOSOPHY. 

the  colors  formed  by  a  prism  are  made  to  pass  through 
a  double  convex  lens  (Fig.  63)  they  will  be  brought 
together  again  and  the  spot  of  light  upon  the  wall  will  be 
white.  The  prism  decomposes  the  white  light  and  brings 

Fig.  C2. 


out  the  colors :  the  convex  lens  combines  the  colors  and 
makes  white  light  again.     Here,  then,  is  a  double  proof 

Fig.  63. 


that  white  light  is  made  up  of  the  seven   colors  of  the 
spectrum. 


LIGHT.  1 95 

How  is  the  rainbow  formed  ? — In  a  shower  of  rain 
each  drop  of  water  is  able  to  decompose  the  sunlight  and 
give  the  different  colors  of  the  spectrum.  This  it  will  do 
if  the  sun  is  shining  brightly  at  the  time  the  drop  is  fall- 
ing, and  you  will  remember  that  all  the  rainbows  you 
ever  saw  were  seen  while  the  sun  was  shining.  When 
the  sun  is  behind  you,  and  a  shower  is  falling  in  front  of 
you,  the  rays  which  pass  through  each  drop  are  decom- 
posed and  the  colors  come  out  in  such  a  direction  that 
some  of  them  enter  your  eye.  Some  drops  send  the  red 
color  to  the  eye  :  others  in  a  different  place  send  orange 
and  others  still  send  yellow;  another  set  gives  blue,  an- 
other indigo,  and  finally  another  violet.  And  these 
seven  colors  are  so  arranged  as  to  form  the  beautiful 
"  bow  of  promise." 

OPTICAL   INSTRUMENTS. 

What  does  Fig.  58  represent? — By  turning  back 
to  Fig.  58,  we  see  that  a  convex  lens  when  held  between 
the  eye  and  a  little  insect  will  help  us  to  see  a  very  large 
image  instead  of  the  little  creature  itself.  Try  it  yourself 
by  taking  grandmother's  spectacles,  if  you  have  no  other 
lens,  and  hold  one  of  the  glasses  just  at  the  right  place, 
which  you  can  find  by  moving  it  back  and  forth  between 
your  eye  and  the  page  of  the  book.  The  letters  will  look 
much  larger  than  they  really  are. 

A  double  convex  lens  used  in  this  way  is  called  a 
simple  microscope. 

This  little  instrument,  by  making  every  little  thing 
look  larger  than  it  is,  becomes  a  very  pleasant,  and  at  the 
same  time  a  very  useful,  instrument  to  every  body.  Most 
people  use  it  for  viewing  tine  engravings  and  in  look- 


126 


NATURAL  PHILOSOPHY. 


Fig.  64. 


ing  at  photographs.  The  watch-maker  uses  it  to  examine 
the  minute  parts  of  his  work,  and  the  jeweler  uses  it  also 
for  the  same  purpose. 

What  is  the  compound  microscope? — The  com- 
pound microscope  is  an  instrument  by  which  to  see  the 
images  of  objects  which  are  so  very  small  that  the  eye 
alone  may  not  see  them  at  all. 

Describe  it. — It  contains  more  than  one  lens;  in  its 
simplest  form  it  has  two.  We  can  understand  it  best  by 
studying  Fig.  04.  Let  us  begin  at  the  bottom,  and  notice 
first  a  concave  mirror.  "We  see  the  rays 
of  sunlight  which  fall  on  this  mirror  are 
thrown  upward  and  brought  together  at 
a.  Now  the  little  object  to  be  magni- 
fied is  placed  at  this  point  and  the 
bright  light  which  goes  up  from  it  must 
pass  through  the  lens  Z>,  which  is  very 
near  to  it.  This  lens  would  magnify  the 
object,  but  not  enough,  and  BO  the  light 
after  going  through  it  is  made  to  go 
through  another  larger  lens  B,  and  then 
into  the  eye  of  the  person.  The  little 
thing  at  the  point  a  is  made  to  look 
large  enough  to  fill  all  the  space  between 
C  and  D. 

What  are  the  lenses  called  ?— The 
lens  1)  is  called  the  object-glass,  and  the 
other,  near  the  eye,  is  called  the  eye-piece. 
The  eye-piece  is  often  made  of  two  lenses 
and  the  object-glass  sometimes  of  as 
many  as  eight. 

How  much  will  this  instrument  magnify? — By 
this  instrument  we  are  able  to  make  the  diameter  of  the 


LIGHT. 


12' 


image  2,000  times  greater  than  the  real  diameter  of  the 
object,  and  in  that  case  the  surface  of  the  image  would  be 
4,000,000  times  as  large  as  that  of  the  object  examined ! 
''Under  such  a  power  a  hair  would  appear  about  six 
inches  thick,  a  fine  needle  would  look  like  a  street-postj 
and  a  grain  of  sand  like  a  mass  of  rock."  Such  power  is 
only  necessary  in  examining  the  very  smallest  objects. 
All  common  preparations  are  best  examined  with  a  power 
which  makes  the  diameter  appear  to  be  only  500  or  600 
times  larger  than  it  really  is. 

What  has  the  microscope  revealed  ?— This  instru- 
ment has  made  known  a  world  of  little  things  around  us 
which  no  human  eye  could  ever  see  without  its  help. 
Little  animals,  and  little  plants,  so  very  small  that  thou- 
sands of  them  together  would  not  be  as  large  as  the  small- 
est particle  of  dust  you  ever  saw,  are  almost  everywhere 
in  the  soil  and  water  and  other  substances  around  us. 

What  is  a  telescope  ? — The  telescope  is  an  instru- 
ment by  which  we  are  able  to  examine  objects  which  are 
so  far  away  that  the  eye  alone  can  not  see  them  distinctly. 
It  contains  lenses  or  mirrors  by  which  the  images  of  dis- 
tant objects  are  made  near  to  the  eye. 

Describe  one  kind. — We  can  describe  one  kind  of 


128 


NATURAL  PHILOSOPHY. 


telescope  best  by  means  of  the  foregoing  diagram,  Fig  C5. 
A  large  convex  lens  is  in  one  end  of  a  tube  and  a  smaller 
one  is  at  the  other  end.  This  small  one  is  the  eye-yluw, 
and  can  be  moved  back  and  forth  so  as  to  be  fixed  at  just 
the  right  distance  from  the  other.  The  light  from  a 
distant  body  coming  through  the  large  lens  forms  an 
image  at  a  J,  and  then  a  person  looking  through  the  eye- 
glass sees  this  image  magnified  at  A'  B'. 

Fig.   06. 


The  tube  containing  these  glasses  is  mounted  in  some 
way  to  allow  it  to  be  pointed   toward  any  object  in  the 


LIGHT.  12;) 

heavens.  The  picture,  Fig.  60,  shows  a  small  one.  One 
of  the  largest  of  this  kind  of  telescope  in  the  world  is  at 
Harvard  College.  Its  object-glass  is  about  eighteen  inches 
in  diameter. 

What  has  the  telescope  revealed  ?— The  telescope 
has  made  known  a  great  many  things  about  the  sun  and 
moon  and  stars.  It  has  shown  that  the  moon  is  covered 
with  mountains  and  valleys ;  and  that  the  sun  has  im- 
mense black  spots  on  its  surface  that  looks  to  us  so  bright. 
It  shows  that  there  are  hosts  of  stars  in  the  sky,  which 
could  never  have  been  seen  without  its  help,  some  of  them 
being  so  far  away  that  light,  travelling  fast  enough  to  go 
around  the  world  about  seven  times  a  second,  would  need 
many  hundreds  of  years  to  come  from  them  to  us. 


HEAT. 


What  is  the  chief  source  of  heat  ? — From  the  sun 
more  heat  is  received  than  from  all  other  sources  together. 
It  is  more  than  90,000,000  of  miles  from  the  earth,  and 
yet  there  comes  out  through  that  vast  distance  a  constant 
flood  of  heat  which,  if  withdrawn  for  a  single  year,  would 
leave  the  whole  earth  in  a  degree  of  cold  which  even  the 
Arctic  regions  never  had. 

What  is  a  source  of  artificial  heat  ?— Combustion 
is  the  chief  source  of  artificial  heat.  Wood,  coal,  or  other 
fuel  burning  in  our  stoves  or  furnaces  warms  our  dwell- 
ings, cooks  our  food,  and  makes  the  steam  by  which  our 
machinery  is  driven.  Next  to  the  sun,  combustion  is  cer- 
tainly the  most  important  source  of  heat. 

How  is  the  heat  in  combustion  produced? — If 
you  shut  the  draught  of  an  "  air-tight"  stove  the  iire  will 
go  out;  or,  if  you  put  a  lighted  candle  under  the  receiver 
of  an  air-pump,  it  will  die  away  when  the  air  is  exhausted. 
We  learn  from  such  experiments  that  no  fuel  can  burn 
without  air.  Unless  air  can  pass  over  the  hot  fuel  in  the 
stove  there  can  be  no  fire. 

Now  the  air  is  made  up  of  two  parts  which  the  chemist 
calls  oxygen  and  nitrogen,  and  it  is  the  oxygen  of  the  air 
passing  over  the  fuel  which  causes  the  combustion.  The 
oxygen  unites  itself  to  the  carbon  and  other  materials  of 


HEAT.  131 

which  the  fuel  consists,  and  to  this  action  the  heat  of  the 
fire  is  due. 

What  is  another  source  of  heat  ? — Mechanical  ac- 
tion, such  as  rubbing  or  pounding,  will  produce  heat. 
Let  the  fingers  be  pressed  down  upon  the  table,  and  then 
smartly  rubbed  back  and  forth :  the  heat  caused  by  this 
friction  will  be  quickly  felt.  Or  if  a  small  cord  or  a 
thread  is  swiftly  drawn  through  the  hand  which  holds  it 
tightly,  the  hand  will  be  cruelly  burned.  If  two  pieces 
of  wood  are  rubbed  upon  each  other  briskly  enough  they 
may  be  set  on  fire ;  in  this  way  savage  people  are  said  to 
have  kindled  their  fires :  more  civilized  people  now  do  it 
more  easily  by  tim ply  rubbing  the  end  of  a  match. 

Blows  also  produce  heat,  as  any  one  may  easily  prove 
by  pounding  a  bullet  with  a  hammer,  fur  he  can  soon 
make  it  too  hot  to  be  comfortably  held  in  the  hand. 

Does  heat  pass  from  one  body  to  another  ? — From 
every  heated  body  rays  of  heat  are  continually  going 
away.  This  is  almost  too  familiar  to  need  illustration,  for 
the  stove  gives  its  warmth  to  all  other  objects  in  the  room, 
and  a  red-hot  cannon-ball  will  part  with  its  heat  so  rapidly 
as  to  very  soon  get  dark  and  finally  cold. 

But  there  is  this  curious  fact  to  add  to  what  has  just 
been  said  :  no  body  is  at  any  time  so  cold  that  it  is  not 
giving  off  heat  to  every  other  around  it.  Heat  is  con- 
stantly passing  away  from  every  body,  no  matter  how  cold 
it  may  already  be,  and  what  is  given  off  by  one  is  being 
received  by  others  in  its  neighborhood,  so  that  it  is  true 
that  even  a  block  of  ice  is  giving  heat  to  a  red-hot  stove, 
if  placed  in  its  vicinity. 

Then  -why  do  hot  bodies  grow  colder  ?— Now  if 
the  ice  and  the  stove  in  this  illustration  should  each  give 


132  NATURAL  PHILOSOPHY. 

off  just  as  much  heat  as  it  gets  from  the  other  back  again, 
the  ice  would  not  melt  nor  the  stove  grow  cold.  But  the 
hot  stove  is  giving  off  much  more  than  it  gets,  and  on  this 
account  it  becomes  gradually  colder  if  the  tire  is  not  kept 
np,  while,  at  the  same  time,  the  ice  gets  much  more  than 
it  gives,  and  is  of  course  melted  by  it. 

A  body  gets  warmer  only  when  it  is  getting  heat  from 
others  faster  than  it  is  giving  heat  to  them :  it  gets  colder 
only  when  it  gives  heat  faster  than  it  gets  it. 

How  does  heat  get  from  one  body  to  another  ? — 
Heat  travels  outward  from  a  hot  body  in  waves  something 
like  the  motion  of  water-waves  when  a  pebble  is  thrown 
into  a  pond  or  lake.  As  the  pebble  puts  the  water  in 
motion,  so  the  hot  body  gives  motion  to  the  substance 
which  fills  the  space  around  it ;  and  as  the  waves  of 
water  spread  outward  in  all  directions  from  the  pebble,  so 
the  waves  of  heat  spread  in  all  directions  from  their 
source.  These  waves  of  heat  warm  every  body  against 
which  they  strike. 

With  -what  velocity  do  these  -waves  travel  ?— 
And  they  go  from  one  body  to  another  so  very  swiftly 
that  one  can  not  measure  the  small  instant  of  time  they 
take  to  pass  through  any  common  distance.  They  start 
from  a  hot  stove  and  at  the  same  instant  they  seem  to 
strike  the  face  of  a  person  in  the  most  distant  corner  of  the 
room.  Indeed,  their  velocity  is  so  great  that  they  would 
be  able  to  go  quite  around  the  world  as  many  as  seven 
times  in  a  single  second !  The  velocity  of  heat  is  like 
that  of  light :  together  they  come  to  us  from  the  sun,  a 
distance  of  more  than  90,000,000  miles,  in  about  8  min- 
utes. This  would  be  at  the  rate  of  about  190,000  miles  a 
second. 

What  name  is  given  to  heat  sent  off  by  bodies  in 


HEAT.  133 

this  way  ? — The  heat  which  travels  in  this  way  is  called 
radiant  heat.  Its  peculiarities  are,  when  briefly  stated, 
first,  it  goes  in  straight  lines ;  second,  it  goes  in  all  possi- 
ble directions  from  its  source ;  and  third,  it  moves  with 
very  great  velocity. 

This  mode  of  transferring  heat  from  place  to  place  is 
called  radiation. 

Is  there  another  mode  ? — Heat  does  not  always 
travel  in  this  way.  If  you  take  one  of  grandmother's  knit- 
ting needles  and  hold  one  end  of  it  in  a  lamp-flame  you  will 
feel  the  other  very  soon  getting  warm.  The  heat  enters  the 
metal  at  one  end  and  travels,  step  by  step,  from  one  parti- 
cle to  another  until  at  length  it  reaches  the  fingers.  In  this 
way  it  is  carried  from  one  part  of  a  body  to  another,  or  it 
maybe  from  one  body  to  another,  if  they  touch  each  other. 
This  mode  of  transferring  heat  is  called  conduction. 

Do  all  solids  conduct  heat  alike?— If  we  take 
two  wires  of  equal  size  and  length,  one  being  of  copper 
and  the  other  of  iron,  and  place  one  end  of  each  in  a 
flame,  we  shall  find  that  the  heat  travels  through,  the  cop- 
per to  the  other  end  quicker  than  through  the  iron. 
Copper  conducts  heat  better  than  iron  does.  A  rod  of 
glass  may  be  melted  within  an  inch  of  the  fingers  that 
hold  it  without  burning  them,  and  a  splinter  of  wood  may 
be  held  in  the  same  way  while  it  burns  to  ashes. 

We  thus  learn  that  each  solid  has  a  rate  of  its  own  at 
which  it  may  conduct  heat.  Liquids  scarcely  conduct  it. 
at  all,  and  gases  in  a  degree  still  less. 

Bodies  that  conduct  heat  freely  are  called  good  con- 
ductors, but  those  that  do  not  are  called  poor  conductors 
or  non-conductors. 

Are  liquids  and  gases  conductors  of  heat  ? — 
Water  is  BO  very  poer  a  conductor  of  heat  that  if  you  put 


134 


NATURAL  PHILOSOPHY. 


ice  at  the  bottom  of  a  glass  vessel  and  then  apply  heat  to 
the  water  above  it,  Fig.  67,  you  may  make  the  water  boil 


Fig.  67. 


Fig 


135 


without  melting  the  ice ;  the  heat  wrill  not  travel  down- 
ward through  the  water  to  the  ice.  Other  liquids,  except 
mercury.,  are  like  water  in  being  very  poor  conductors  of 
heat. 

Gases  are  still   poorer  Fi?-69- 

conductors  than  liquids. 

What  is  one  effect 
caused  by  heat? — Let 

us  learn  by  experiment 
what  effect  heat  pro- 
duces : 

1st.  In  solids.     A  ball 
of  iron  or  of  brass  is  taken 
just  large  enough  to  pass 
easily  through  a  ring  of 
the  same  material.     The 
ball  is  then  heated  by  a 
lamp,  after  which  it  will 
be  too  large  to  go  through 
the    ring.     It    will    rest 
upon   the  ring  (Fig.  68) 
until   it  gets  cold  again, 
when  it  once  more  passes 
easily    as    at    first.     We    j 
see  that  heat  makes  this  -Sjjj 
ball  larger.     And  it 
the    same     effect     upon  ~ 
other  solids. 

2d.  Liquids.  A  glass  bulb  with  a  long  open  stem  is 
-used.  The  bulb  is  filled  with  water  and  the  stem  partly 
filled,  after  which,  if  the  bulb  is  plunged  into  hot  water 
(Fig.  69),  the  water  in  the  stem  will  be  seen  slowly  rising, 


136 


NATURAL   PHILOSOPHY. 


Fijr.  70. 


until  perhaps  it  will  run  over  the  top.  The  water  grows 
larger  as  it  gets  warmer.  We  see  that  heat  expands  this 
liquid  :  it  does  the  same  thing  for  others. 

3d.  Gases.  Fig.  70  shows  an  experiment  with  air. 
The  glass  bulb  with  its  long  open 
stem  is  used  for  this  also.  The 
little  black  spot  near  the  end  of 
the  tube  represents  a  little  drop 
of  ink  which  has  been  put  into 
the  tube  and  which  will  be  held 
there  by  the  walls  of  glass.  Now 
when  the  warm  hands  take  hold 
of  the  bulb  the  drop  will  run  up 
still  higher  in  the  tube.  The  air 
below  it  pushes  the  ink  up  be- 
cause it  wants  more  room.  The 
heat  of  the  hand  makes  the  air 
larger  than  it  was.  Heat  also 
expands  all  other  gases. 

"We  learn  from  these  experi- 
ments that  the  general  effect  of 
heat  is  to  expand  all  bodies  to 
which  it  is  applied. 

What  facts  illustrate  the 
expansion  of  solids  ?— An  iron 
gate  which  opens  and  shuts  easily 
in  cold  weather,  will  stick,  in 
a  warm  day,  owing  to  the  heat  which  expands  it.  Pipes 
of  cast-iron  for  conveying  hot  water  are  longer  when  full 
than  when  empty.  It  is  said  that  an  ignorant  man  once 
tried  to  warm  a  large  manufactory  by  steam.  He  laid 
one  large  iron  pipe  from  the  boiler  to  the  farther  end  of 
the  building,  and  then  passed  branches  from  this  through 


HEAT.  137 

holes  into  the  several  rooms.  The  very  first  time  he 
filled  the  pipes  with  steam,  the  expansion  of  the  main 
pipe  tore  it  away  from  all  its  brandies  ! 

The  rails  of  a  railroad-track  are  longer  in  summer  than 
in  winter. 

What  facts  illustrate  the  expansion  of  liquids  ?— 
A  kettle  nearly  full  of  cold  water  will  be  quite  full  when 
the  water  is  heated :  the  water  will  run  over  long  before 
it  boils. 

Twenty  gallons  of  alcohol  in  midwinter  will  become 
about  twenty-one  gallons  in  midsummer.  Hence  cunning 
dealers  try  to  make  purchases  in  winter  and  sales  in 
summer,  that  the  heat  of  summer  may  add  to  their  profits. 

Does  heat  always  expand  water? — At  all  tem- 
peratures above  39°  water  will  be  expanded  by  applying 
heat,  but  at  temperatures  below  39°  water  will  be  con- 
tracted by  applying  heat.  At  39°  a  given  weight  of  water 
is  as  small  as  it  can  be ;  heat  it  or  cool  it  as  you  will,  and 
it  will  be  expanded. 

Are  all  liquids  like  water  in  this  respect  ? — To 
show  how  different  are  the  effects  of  a  change  of  tempera- 
ture in  water  and  other  liquids,  the  following  experiment  is 
made.  Three  glass  globes  with  long  necks  are  placed  in 
a  large  dish  nearly  filled  with  ice-cold  water  (Fig.  71). 

Suppo'se  the  water  cooled  to  32°?— At  32°  the 
water  freezes.  The  expansion  at  this  moment  is  greater 
than  at  any  moment  before,  so  that  the  ice  is  larger  than 
the  water  from  which  it  is  made. 

On  this  account  pitchers  and  water-pipes  are  often 
broken  by  the  water  freezing  in  them. 

Ice  being  larger  must  also  be  lighter  than  the  water 
from  which  it  is  made.  Were  it  not  for  this  fact  our 
ponds  and  rivers  would  never  be  covered  with  a  blanket 


138  NATURAL  PHILOSOPHY. 

of  ice  as  now  they  are  in  the  winter.  The  ice,  instead, 
would  sink  to  the  bottom  as  fast  as  formed.  There  could 
be  no  skating  then,  you  notice ;  but  that  would  not  be  the 

Fig.  7L 


saddest  of  the  story,  for  there  would  soon  be  no  human 
beings  to  enjoy  that  or  any  other  sport.  As  it  now  is,  the 
ice  stays  on  top  and  keeps  the  water  from  freezing  to  any 
great  depth  ;  but  if  it  should  sink,  it  would  go  on  forming 
until  the  whole  body  of  water  would  become  ice  from 
bottom  to  top,  and  then  the  atmosphere  would  get  colder 
and  colder,  until  neither  plants  nor  animals  could  live 
at  all. 

What  facts  illustrate  the  expansion  of  air  ? — The 
snapping  of  wood  in  the  tire  is  caused  by  the  expansion 
of  air.  The  air  in  the  pores  of  the  wood,  suddenly  heated, 
expands  and  bursts  the  wood  witli  a  sharp  report, 

The  first  balloons  that  were  made  were  filled  with  hot 
air,  and  they  went  up  toward  and  even  above  the  clouds, 


HEAT.  139 

because  the  air  they  contained  was  expanded  and  made 
lighter  than  the  cold  air — so  light  that  it  could  rise  and 
carry  the  balloon  up  with  it.  Only  toy-balloons  are  now 
iil led  with  hot  air ;  those  by  which  men  are  carried  to 
the  clouds  are  tilled  with  common  illuminating  gas,  or 
sometimes  with  hydrogen. 

Which  is  most  expanded  by  heat,  solids,  liquids, 
or  gases  ? — A  little  addition  of  heat  expands  a  gas  very 
much ;  the  same  applied  to  a  liquid  would  cause  an  in- 
crease in  size  difficult  to  see,  and  if  it  were  applied  to 
a  solid,  would  not  change  its  size  enough  to  be  noticed  at 
all. 

Do  all  solids  and  liquids  expand  equally? — Instead 
of  all  solids  expanding  equally,  each  one  has  a  certain 
rate  of  its  own.  Brass  for  example,  will  expand  faster 
than  iron.  Each  liquid  also  has  a  rate  of  its  own. 

Do  all  gases  expand  equally  ? — Gases  all  expand 
alike  by  heat.  Air,  oxygen,  and  all  other  gases,  heated 
alike,  will  expand  equally. 

What  is  a  second  effect  of  heat  ?— If  iron  is  heated 
it  will  go  on  expanding  long  after  it  has  become  red-hot, 
until  finally  it  melts.  The  solid  is  then  changed  into  a 
liquid  ;  this  is  a  second  effect  of  heat.  It  is  called  lique- 
faction. 

Do  all  solids  melt  at  the  same  temperature  ? — A 
few  examples  which  all  have  noticed  will  show  that  solids 
melt  at  very  different  temperatures.  The  warmth  of  the 
hand  will  melt  ice,  but  not  wax.  Sulphur  will  melt  on  a 
hot  stove ;  it  needs  a  temperature  of  230°,  but  iron  does 
not  melt  until  it  is  heated  to  about  3,000°.  Each  sub- 
stance has  a  certain  temperature  at  which  it  melts.  This 
temperature  is  called  the  melting-point. 


140 


NATURAL  PHILOSOPHY. 


What  is  a  third  effect  of  heat  ?— If  water  is  heated 
it  will  go  on  expanding  until  its  temperature  is  212°  ;  at 
this  temperature  it  bolls.  The  liquid  is  then  changed  into 
a  gas ;  this  is  a  third  effect  of  heat.  It  is  called  vaporisation. 
Do  all  liquids  boil  at  the  same  temperature  ? — 
Water  boils  in  an  open  iron  vessel  at  the  temperature  of 
212°,  but  alcohol  will  boil  at  173°,  and  ether  only  needs 
to  be  heated  to  95°.  These  illustrations  show  that  each 
liquid  has  its  own  degree  of  heat  at  which  it  boils :  this 
temperature  is  called  the  boiling-point. 

Describe  the  experiment  in  which,  by  cooling 
the  vessel,  water  is  made  to  boil.— A  very  curious 
experiment  is  represented  in  Fig.  72.     In  the  first  place  a 
FiC.  72.  glass  bulb  with  long  open  stem 

is  partly  filled  with  water.  This 
water  is  then  boiled  for  some  time 
until  the  steam  has  driven  all 
the  air  out  of  the  vessel.  While 
the  water  is  still  boiling,  the 
stem  is  tightly  corked  and  the 
heat  taken  away  at  the  same 
moment.  All  this  is  done  to 
get  rid  of  the  air  and  leave 
nothing  but  water  and  steam  in 
the  vessel.  The  bulb  is  then 
turned  upward  as  in  the  picture. 
After  thisjww  cold  water  upon 
the  bulb,  and  the  water  inside 
will  boil  vigorously.  Stop  pour- 
ing cold  water  and  the  boiling  will  cease,  but  as. often  as 
the  cold  is  applied  the  boiling  will  begin.  This  may  be 
kept  up  until  the  vessel  is  cold  enough  to  be  held  in  the 
hand  without  inconvenience. 


HEAT.  141 

. « 

What  does  the  cold  water  do  ?— Now  all  that  the 
cold  water  does  is  to  cool  the  steam  that  is  in  the  bulb 
and  condense  it  into  water.  By  this  means  the  pressure 
of  the  steam  is  taken  off  from  the  water  inside. 

Describe  another  experiment  in  which  water  is 
made  to  boil  without  fire. — The  pressure  may  be  taken 
from  the  surface  of  the  water  in  another  way.  The  water 
is  put  into  a  flask,  to  the -top  of  which  is  fastened  a  long 
tube.  This  arrangement  is  shown  in  Fig.  73.  The 

Fig.  73. 


water  is  then  heated  until  it  begins  to  boil.  The  flask 
is  then  taken  from  the  fire  and  its  tube  is  fastened  to  the 
plate  of  the  air-pump.  On  working  the  pump,  the  air 
and  steam  are  taken  from  the  flask,  and  the  water,  which 
by  this  time  is  much  below  212C\  begins  to  boil  violently. 
What  do  these  experiments  teach? — We  learn 
from  these  experiments  that  by  taking  pressure  away  from 


142  NATURAL  PHILOSOPHY. 

the  surface  of  water,  boiling  will  go  on  at  a  lower  temper- 
ature. 

In  the  open  air  water  boils  at  212C,  but  the  pressure  of 
the  atmosphere  is  151bs.  upon  each  square  inch  of  surface. 
If  it  were  not  for  this  pressure  water  would  boil  at  a  tem- 
perature much  lower  than  212°,  indeed  at  a  temperature 
not  much  above  that  of  a  hot  summer  day. 

Water  boils  at  a  low  temperature  on  top  of  high  mount- 
ains. In  fact,  at  places  very  high  above  the  level  of  the 
sea,  boiling  water  is  not  hot  enough  to  cook  meat,  or  even 
to  boil  eggs,  because  the  pressure  of  the  atmosphere  is  so 
much  less  than  at  the  sea-level. 

Suppose  the  pressure  is  increased  ?— On  the  other 
hand,  if  water  is  heated  under  a  greater  pressure  than  that 
of  the  atmosphere,  the  boiling-point  will  be  higher  than 
212°.  In  a  word,  the  boiling-point  is  mixed  by  increasing 
the  pressure  and  lowered  by  lessening  it. 

Does  steam  exert  pressure  ?— Steam  often  lifts  the 
lid  of  a  kettle  in  order  to  make  its  escape,  and  when 
confined  in  a  boiler,  it  sometimes  bursts  the  stoutest 
bands  of  iron,  killing  people  and  destroying  buildings  by 
the  force  of  the  explosion.  Such  facts  show  that  steam 
can  exert  a  great  pressure  when  confined. 

How  can  it  be  used  to  move  machinery  ? — If 
there  is  a  piston  moving  freely  inside  of  a  cylinder,  and 
then  if  steam  is  let  into  the  cylinder,  first  at  one  end  and 
then  at  the  other,  its  expansive  force  or  pressure  will 
knock  the  piston  back  and  forth  from  one  end  to  the 
other  with  great  rapidity  and  power.  The  piston  may 
move  a  crank  which  turns  a  wheel,  and  then  by  bands  or 
cogs  this  wheel  may  turn  other  wheels.  In  this  way 
steam  is  made  to  move  machinery.  This  is  the  principle 
of  the  xteam-engi'ne,  by  which  ships  are  driven  over  the 


HEAT.  143 

sea  and  railroad  trains  across  the  continent,  and  by 
which  so  much  of  all  the  machinery  in  the  world  is 
moved. 

Can  water  be  heated  above  212°  ? — If  water  is 
slowly  heated  its  temperature  will  rise  until  the  liquid 
boils  at  212°  :  after  that  the  water  grows  no  hotter.  The 
tire  may  be  quickened  and  the  boiling  will  be  more  vio- 
lent, but  the  water  will  not  become  any  hotter.  This  is 
always  true  when  the  water  is  heated  in  open  vessels 
such  as  are  generally  used. 

What  becomes  of  the  heat  added  to  the  boiling 
•water?— Now  the  tire  gives  heat  to  the  boiling  water 
all  the  time,  but,  as  we  see,  does  not  make  it  any  hotter. 
All  the  heat  that  goes  into  the  water  is  then  used  in 
changing  the  wafer  into  steam. 

Can  -we  get  this  heat  back  again  ? — All  this  heat 
which  has  been  used  to  change  the  water  into  steam,  will 
be  given  up  when  the  steam  changes  back  into  water. 
This  is  the  reason  that  a  plate  grows  hot  so  very  quickly 
when  held  in  the  steam  that  issues  from  the  spout  of  the 
tea-kettle. 

Is  this  principle  ever  applied?  —  Buildings  are 
sometimes  warmed  by  steam.  From  a  large  steam-boiler 
cast-iron  pipes  are  laid  to  the  many  rooms  to  be  wanned, 
and  the  steam  is  forced  through  their  entire  length.  The 
steam  is  condensed  in  going  through  the  cold  pipes,  and 
gives  up  to  them  the  heat  which  it  took  from  the  fire. 
They  very  soon  become  very  warm,  and  warm  the  air 
which  is  in  contact  with  them. 

Do  we  know  the  temperature  of  bodies  by  feel- 
ing them?— Let  ns  take  three  vessels  of  water,  one  almost 
as  cold  as  ice,  another  just  warmer  than  the  ha^d,  and  a 


144  NATURAL  PHILOSOPHY. 

third  as  hot  as  the  hand  can  bear.  Let  one  hand  be  held 
in  the  first  vessel  of  cold  water  and  the  other  in  the  vessel 
of  hot  water  for  a  while,  and  then  let  both  be  plunged 
into  the  vessel  of  warm  water.  It  will  be  found  that  to 
one  hand  the  water  is  cold,  to  the  other  it  is  hot,  at  the 
same  time.  Of  course,  by  the  feeling  we  could  not  tell 
whether  it  is  really  hot  or  cold. 

Give  another  example. — An  oil-cloth  and  a  carpet, 
where  they  lie  together  upon  the  floor,  are  of  the  same 
temperature,  but  the  oil-cloth  will  feel  cold  and  the  carpet 
warm  to  the  hand  at  the  same  time. 

The  reason  of  this  is  that  the  oil-cloth  is  a  better  con- 
ductor of  heat  than  the  carpet,  and  takes  the  heat  of  the 
hand  away  faster,  so  that  the  hand  grows  cold  quicker 
when  upon  it. 

By  what  instrument  can  we  find  the  tempera- 
ture of  bodies  ? — The  instruments  by  which  to  measure 
temperature  are  called  thermometers.  The  common  ther- 
mometer contains  mercury,  which  expands  when  heated, 
and  contracts  when  cooled,  and  by  these  changes  of 
volume  shows  the  temperature. 

Describe  the  common  thermometer.— The  ther- 
mometer is  very  common,  and  a  single  look  at  it  would 
be  better  than  any  description  of  it  can  be.  However,  it 
ma}7  be  described  as  a  glass  tube,  with  a  bulb  at  one  end, 
while  the  other  end  is  shut  air-tight,  containing  mercury 
which  fills  the  bulb  and  part  of  the  stern,  and  having  a 
scale  behind  the  stem  to  show  the  height  of  the  fluid. 
Fig.  74  shows  two  forms  of  this  instrument. 

How  is  it  used? — By  putting  the  bulb  into  water  or 
any  othei  substance  the  height  of  the  mercury  in  the  stem 
will  show  how  hot  it  is.  Put  the  bulb  in  water,  for  in- 
stance, and  if  the  mercury  rises  in  the  stem  up  to  the 


HEAT. 


145 


place  marked  90  on  the  scale,  then  the 
temperature  of  the  water  is  90°. 

How  is  Fahrenheit's  thermometer 
graduated  ?— The  place  where  the 'mer- 
cury stands  when  the  bulb  is  immersed  in 
boiling  water  is  marked  212°  on  the  scale ; 
where  the  mercury  stands  when  the  bulb  is 
immersed  in  freezing  water  is  marked  32° : 
the  space  between  these  is  divided  into  180 
equal  parts,  called  degrees,  and  divisions  of 
the  same  size  are  marked  off  on  the  scale 
both  above  and  below  these  points. 

How  is  the  centigrade  thermometer 
graduated  ? — In  the  centigrade  thermom- 
eter, the  place  where  the  mercury  stands 
when  the  bulb  is  placed  in  boiling  water  is 
marked  100° ;  where  it  stands  when  the 
bulb  is  in  freezing  water  is  called  0°,  and 
the  distance  between  is  divided  into  100 
equal  parts  or  degrees. 


s 


MAGNETISM. 


What  is  a  loadstone  ? — Several  hundred  jears  ago 
pieces  of  a  certain  kind  of  iron  ore  were  found  in  the 
earth  which  had  the  power  to  attract  bits  of  iron.  They 
would  lift  needles  or  small  nails  or  other  bits  of  iron 
which  they  touched,  and  hold  them  suspended  in  the  air 
as  if  they  were  cemented  to  the  stone  with  glue.  The 
ore  of  iron  which  has  this  wonderful  power  is  called  the 
loadstone.  It  is  found  sometimes  in  this  country,  but  not 
in  such  abundance  as  in  Sweden  and  Norway,  and  in  some 
parts  of  Asia.  A  good  specimen  may  be  bought  for  a  few 
cents,  and  it  is  an  interesting  and  instructive  toy.  It  is 
often  called  the  natural  magnet. 

What  is  a  magnet  ? — If  a  loadstone  is  rolled  in  iron 
filings  it  will  attract  them  and  hold  them  clinging  to  its 
surface,  but  when  rolled  in  filings  of  brass  or  copper  not 
one  of  these  will  it  pick  up.  It  has  a  curious  preference 
for  iron.  Any  body  that  will  attract  iron  in  preference 
to  other  metals  is  called  a  magnet. 

What  are  artificial  magnets  ? — A  bar  of  steel  may 
be  made  a  magnet  by  simply  rubbing  it  upon  the  load- 
stone. In  this  way  the  blade  of  a  penknife  may  be  given 
the  power  to  pick  up  bits  of  iron,  such  as  small  needles, 
tacks,  or  iron  filings.  One  blade  can  get  this  power  also 
from  another  which  has  been  already  made  magnetic;  and 
what  is  a  little  singular,  perhaps,  is  that  the  one  that  gives 


MAGNETISM.  147 

this  power  to  another  is  none  the  weaker  for  it;  it  is 
even  stronger  than  before.  All  such  pieces  of  steel  are 
magnets. 

Iron  also  may  be  made  magnetic,  but  it  will  not  stay  so 
unless  it  be  first  hardened  more  than  usual. 

In  what  two  shapes  are  magnets  made  ? — ArtiV 
ficial  magnets  are  made  in  two  forms.  They  are  generally 
either  a  straight  bar  of  steel  or  else  a  bar  bent  into  the 
form  of  a  horse-shoe.  The  first  is  called  the  bar  magnet, 
and  the  second  is  called  the  horse-slioe  magnet. 

What  is  an  armature  ? — A  horse-shoe  magnet  gener- 
ally has  with  it  a  bar  of  iron  to  reach  across  from  one  end 
of  the  magnet  to  the  other.  Such  a  bar  is  called  the 
armature, 

In  what  part  of  a  magnet  is  the  attraction 
strongest  ? — By  rolling  a  bar  magnet  in  a  bed  of  iron 
filings  and  then  lifting  it,  the  filings  may  be  seen  clinging 
to  the  ends  of  the  bar  in  Fig.  75. 

curious  tufts  (Fig.  75),  while 
along  the  middle  few  or  none 
will  be  found. 

This  experiment  shows  the  power  of  the  magnet  to  be 
much  greater  at  the  ends  than  elsewhere.  The  ends  of 
the  magnet,  or  the  points  at  which  the  force  is  strongest, 
are  called  the  poles. 

,  Fig.  '76  represents  a  horse-shoe  magnet  with  an  arma- 
ture across  the  poles  holding  up  a  heavy  weight. 

What  is  the  magnetic  needle  ?— A  slender  bar  mag- 
net balanced  upon  a  pivot  (Fig.  77)  is  called  a  magnetic 
needle. 

Lett  to  itself,  such  a  needle  will  always  be  found  point- 
ing toward  the  north  and  south.  It  will  not  rest  in  any 
other  position.  If  you  push  it  out  of  this  direction  it  will 


148 


NATURAL  PHILOSOPHY. 


Fig.  76. 


swing  back  again  the  moment  you  let  go  of  it,  and  after 
vibrating  from  one  side  to  the  other  for  a  time,  it  will  at 
last  come  to  rest  again  with  the  same  end  to  the  north  as 
before. 

The  end  that  points  toward  the 
north  is  called  t\\Q  north  pole :  the 
other  end  is  called  the  south  pole. 
What  use  is  made  of  the 
magnetic  needle  ?  —  What  is 
called  the  mariner's  compass  is  a 
magnetic  needle  placed  over  a  dial 
on  which  are  marked  north,  south, 
east,  west,  and  many  other  direc- 
tions, or  as  they  are  called,  "  points 
of  the  compass."  This  little  in- 
strument is  placed  where  it  will 
be  every  moment  in  view  of  the 
man  who  guides  the  ship,  and  tells 
him  every  moment  in  what  direc- 
tion the  ship  is  going. 

No  matter  how  dark  the  night 
or  how  rough  the  sea  may  be,  the 
faithful  needle,  pointing  always  so 
nearly  north  and  south,  guides  the 
storm-tossed  searm.n  safely  to  his 
port. 

What  is  a  dipping  needle  ? — If  a  magnetic  needle 
is  hung  in  a  way  to  let  its  poles  move  up  and  down  it  will 
not  rest  in  a  horizontal  position.  The  picture  (Fig.  78) 
shows  what  direction  it  will  take :  the  north  pole  will  be 
lower  than  the  other.  A  needle  fixed  in  this  way  is 
called  a  dipping  needle. 

Is  the  dip  of  the  needle  everywhere  alike  ? — The 


MAGNETISM. 


149 


dip  of  the  needle  is  not  the  same  in  all  places  on  the  earth. 
In  the  most  northern  regions  the  needle  is  most  oblique, 


Fig.  77, 


that  is  to  say,  the  dip  is 
greatest.  Just  at  the  north 
pole  the  needle  would  point 
its  north  pole  downward  to 
the  ground.  As  the  needle 
is  carried  farther  to  the  south 
the  north  pole  rises  until 
when  at  the  equator  the  nee- 
dle would  be  horizontal,  or 
have  no  dip  at  all,  and  then 
when  carried  farther  into 
the  southern  hemisphere  the  south  pole  would  dip  instead 
of  the  north  pole. 

Fig.  78 


How   will    one    magnet   act    upon    another  ? 


150  NATURAL   PHILOSOPHY. 

Let  one  magnet  be  brought  near  to  another ;  suppose, 
for  instance,  that  you  hold  one  in  the  hand  and  point  its 
north  pole  toward  the  south  pole  of  the  magnetic  needle. 
They  will  quickly  come  together :  if  in  reach  of  eacli 
other  they  can  not  be  kept  apart.  These  unlike  poles  at- 
tract each  other. 

Next  point  the  north  pole  toward  the  north  pole  of  the 
needle,  and  it  will  swing  quickly  away,  so  that  it  will 
be  almost  impossible  to  make  the  two  magnets  touch  each 
other.  These  poles  of  the  same  name  repel  each  other. 

State  the  law  of  attraction  and  repulsion  ? — It 
will  always  be  the  case  as  in  the  experiments  just  de- 
scribed, that  poles  of  unlike  names  attract  each  oilier, 
while  poles  of  the  same  name  repel  each  other. 

Describe  the  experiment  to  illustrate  induction. 

— Hold  a  strong  magnet  in  a  vertical  position  and  touch 
the  lower  pole  with  the  end  of  a  much  smaller  bar  of 
iron  :  the  magnet  will  hold  it  h'rmly  in  the  air.  Another 
smaller  bar  may  be  hung  from  the  lower  end  of  the  first, 
and  another  yet  from  it.  The  first  bar  of  iron  receives  its 
magnetism  from  the  magnet,  and  then  the  second  from 
the  first  and  the  third  from  the  second.  This  power  of  a 
magnet  to  impart  magnetism  to  other  bars  of  iron  or  steel 
is  called  induction. 

Will  induction  occur  when  the  magnet  does  not 
touch  the  iron  ? — We  may  cover  the  end  of  the  magnet 
with  paper  so  that  the  bar  of  iron  can  not  touch  the  polo, 
and  vet  find  that  it  becomes  a  magnet  by  induction  as 
before. 

Or  we  may  show  this  same  thing  by  another  and  more 
curious  experiment.  Let  a  horse-shoe  magnet  be  placed 
poles  upward,  and  lay  across  its  ends  a  piece  of  stiff  card- 


MAGNETISM.  151 

board  or  a  piece  of  glass.  Sprinkle  iron  filings  upon  the 
cardboard  and  at-  the  same  time  gently  tap  it  with  the 
linger.  The  filings  will  then  be  seen  to  collect  in  clusters 
around  the  poles  of  the  magnet  and  to  arrange  themselves 
in  strange  curves  from  pole  to  pole. 

Now  in  this  experiment  each  little  filing  becomes  a 
magnet  by  induction  through  the  card  or  glass,  and  then 
each  pole  of  one  attracts  the  opposite  pole  of  its  neighbor, 
so  that  they  cling  to  each  other  in  curves  and  clusters 


ELECTRICITY  BY  FRCITION. 


Describe  the  experiment  with  the  glass  rod. — 

Fig.  79  shows  the  results  of  an  easy  and  amusing  ex- 
periment. A  glass  rod,  or  perhaps  a  stick  of  sealing-wax, 
must  be  rubbed  briskly  with  a  flannel  cloth  tor  a  few 

Fig.  TO. 


moments,  and  then  held  near  to  pieces  of  some  light  sub- 
stance, such  as  bits  of  cotton  or  balls  of  pith  taken  from 
the  elder-bush  or  corn-stalk.  These  light  bodies  will 
quickly  jump  upward  against  the  rod,  and  then,  as  if  dis- 
appointed with  their  visit,  as  quickly  jump  away  again. 


ELECTRICITY   BY  FRICTION.  l.-j3 

What  does  this  experiment  show?— We  see  by 

this  experiment  that  rubbing  glass  with  flannel  gives  to 
the  glass  a  power  which  it  did  not  have  before — the  power 
to  attract  and  to  repel  light  substances. 

What  is  this  power  called? — This  new  power 
aroused  in  the  glass  is  called  electricity.  In  this  case  the 
electricity  is  produced  by  friction. 

Does  friction  always  produce  electricity? — 
Whenever  substances  are  rubbed  together  electricity  is 
evolved.  Arid  yet  if  an  iron  rod  is  used  in  place  of  the 
glass  in  the  experiment  (Fig.  79),  the  pith  balls  will  not 
stir  from  the  table,  because  the  electricity  flies  along  the 
surface  of  the  iron  and  away  through  the  hand  as  fast  as 
it  is  produced. 

What  are  conductors  and  non-conductors? — All 
bodies  which  will  allow  electricity  to  pass  over  their  sur- 
faces freely  are  called  conductors  of  electricity.  Iron  is  a 
good  conductor,  and  so  are  other  metals  and  many  com- 
mon substances  besides. 

Bodies  which,  like  glass,  will  not  allow  electricity  to  pass 
freely  over  them  are  called  non-conductors.  Besides  glass 
many  other  common  substances  are  non-conductors.  Air 
is  one  of  the  most  perfect  of  them  all.  And  among  others 
it  is  well  to  mention  India-rubber,  sealing-wax,  and  silk. 

What  is  an  electroscope  ? — An  instrument  to  detect 
the  presence  of  electricity  in  any  body  is  called  an  electro- 
scope. Fig.  80  will  give  a  good  idea  of  one  of  these  instru- 
ments. It  is  only  a  little  ball  of  pith  hung  by  a  silk  cord 
from  the  end  of  a  standard. 

The  glass  rod,  after  being  rubbed  with  the  flannel  cloth, 
will  show  its  electricity  by  attracting  the  pith-ball.     Chi 
coming  in  contact  with  the  glass  the  pith  itself  becomes 
electrified,  and  then  jumps  away  from  the  glass. 
7* 


154 


NATURAL  PHILOSOPHY. 


What  is  an  electric  machine  ?— The  glass  rod  and 
sealing-wax  will  give  electricity  enough  only  to  show 
itself  distinctly.  When  it  is  to  be  obtained  in  greater 

Fig.  80. 


force  other  apparatus  must  be  used.  Any  apparatus  l>y 
which  electricity  of  considerable  force  is  obtained  may  be 
called  an  electrical  machine. 

The  most  common  form  of  the  electrical  machine  con- 
sists of  a  large  circular  glass  plate,  with  its  axle  resting 
upon  pillars.  This  plate  is  turned  with  a  crank,  and  in 
turning  it  rubs  between  two  rubbers.  This  friction  gives 
the  electricity.  Then  there  is  a  brass  ball  or  cylinder 
resting  upon  a  glass  pillar  which  takes  the  electricity  from 
the  glass  plate.  This  ball  or  cylinder  is  called  the  prime 
conductor,  and  the  electricity  for  experiments  is  taken 
from  it. 


ELECTRICITY   BY  FRICTION.  155 

In  what  two  ways  does  electricity  act? — The 

experiment  with  the  electroscope  described  a  little  while 
since,  shows  that  electricity  acts  both  by  attraction  and 
repulsion.  Look  back,  and  read  that  experiment  again. 

Will  glass  and  sealing-wax  act  alike  ?— The  elec- 
troscope will  help  us  to  show  that  the  electricity  from 
glass  and  that  from  sealing-wax  do  not  act  alike. 

Let  the  glass  rod  be  rubbed  and  once  brought  in  con- 
tact with  the  pith-bail  (Fig.  80).  The  ball  will  after  this 
be  repelled  by  the  glass.  Next  rub  a  stick  of  sealing-wax, 
and  then  hold  it  near  to  the  pith :  the  little  ball  will 
quickly  fly  toward  it,  being  attracted  by  the  sealing-wax. 
Do  not  let  it  touch  the  sealing-wax,  and  you  will  find  that 
every  time  the  glass  comes  near  it  the  ball  will  be  r<-pell<?d, 
and  every  time  the  wax  approaches  it  the  ball  will  be 
attracted.  The  glass  and  sealing-wax  act  in  exactly  oppo- 
site ways. 

How  are  these  two  actions  named? — The  elec- 
tricity of  the  glass  has  been  called  positive  electricity,  and 
that  from  the  sealing-wax  has  been  called  neyative  elec- 
tricity. 

Now,  whenever  other  non-conductors  are  rubbed,  some 
of  them  will  give  positive  electricity  and  others  negative 
electricity.  But  when  we  speak  of  positive  electricity  we 
mean  simply  that  it  is  like  the  electricity  from  glass,  and 
the  term  negative  electricity  means  only  that  the  elec- 
tricity is  like  that  obtained  from  sealing-wax. 

What  is  the  law  of  attraction  and  repulsion  ? — 
The  action  of  these  two  forces  is  always  in  obedience  to 
the  following  law : 

Bodies  having  the  same  kind  of  electricity  repel  each 
other,  but  having  opposite  kinds  they  attract  each  other. 


156  NATURAL  PHILOSOPHY. 

« 

Show  that  electricity  is  only  on  the  outside 
surface  of  a  body. — Fig.  81  shows  a  curious  experiment 
which  teaches  us  where  the  electricity  of  a  body  is  to  be 

Fig.  81. 


found.  A  sack  made  of  muslin  and  in  the  shape  of  a 
cone  is  fastened  to  a  metallic  ring  upon  a  standard.  The 
sack  is  to  be  held  out  by  a  long  silk  cord.  When  the 
ring  is  brought  in  contact  with  any  body  already  charged 
with  electricity,  the  force  will  spread  into  the  ring  and 
from  it  into  the  sack.  On  examining  the  sack  its  outside 
surface  will  show  the  presence  of  electricity,  but  its  inside 
surface  will  not.  Now  by  taking  hold  of  the  other  end 
of  the  cord  the  sack  may  be  turned  inside  out ;  and,  on 
examining  again,  the  electricity  is  found  to  be  on  what  is 
now  the  outside,  while  not  a  trace  of  it  can  be  found  upon 
the  inside.  No  matter  how  often  nor  how  quickly  the 
sack  is  turned  inside-out,  the  electricity  will  always  be 
found  on  the  outside. 

Can  a  conductor  be  charged  with  electricity  ? — 

Since  electricity  will  pass  freely  over  the  surface  of  any 


ELECTRICITY  BY  FRICTIOX.  157 

conductor,  it  would  at  first  seem  to  be  impossible  to  make 
it  remain,  or,  in  other  words,  to  charge  the  conductor. 
But  k-t  the  conductor  be  placed  upon  a  glass  support,  and 
the  electricity  will  have  no  means  of  escape,  so  that  it  will 
be  compelled  to  stay. 

A  body  which  does  not  touch  any  other  conducting  sur- 
face is  said  to  be  insulated.  The  electricity  can  not  stay 
on  the  surface  of  any  conductor  unless  it  be  insulated. 

Can  a  pointed  conductor  be  charged? — A  con- 
ductor with  a  pointed  wire  reaching  out  from  its  surface 
will  not  retain  electricity  even  when  insulated.  The  force 
passes  off  from  the  point  into  the  air,  and  seems  to  be  lost. 

A  candle  flame  held  in  front  of  such  a  point  will  be 
blown  as  if  struck  by  a  breeze  of  air,  and  indeed  it  is,  for 
the  air  electrified  by  the  point  is  repelled  and  moves 
away  :  it  is  this  current  of  air  that  wafts  the  flame. 

Suppose  a  pointed  conductor  brought  near  to  an 
electrified  body. — If  we  put  the  pointed  end  of  a  wire 
near  to  any  body  which  is  electrified  it  will  draw  the 
electricity  away  without  touching  it.  To  point  the  finger 
at  the  prime  conductor  of  an  electrical  machine  will  be 
almost  enough  to  keep  the  conductor  from  being  charged : 
and  an  open  penknife  held  in  the  hand  and  pointed  at 
the  conductor  will  be  found  quite  enough  to  draw  away 
the  electricity  as  fast  as  it  is  evolved. 

For  -what  purpose  are  pointed  conductors  used  ? 
— Lightning-rods  are  pointed  conductors,  which  are  used 
to  protect  buildings  from  being  struck  by  lightning. 

Lightning  is  nothing  more  than  the  light  caused  by 
electricity  in  the  air  or  clouds,  and  thunder  is  only  the 
noi.se  that  is  made  by  the  .electricity  when  it  passes  from 
one  cloud  to  another  or  to  the  earth. 


158  NATURAL  PHILOSOPHY. 

"When  a  cloud  full  of  electricity  floats  along  over  a 
house  the  electricity  sometimes  leaps  into  the  building 
and  tears  it  to  pieces,  or  perhaps  sets  it  on  lire  ;  but  ii'  the 
house  has  a  good  lightning-rod  reaching  above  the  roof, 
the  point  of  the  rod  will  take  the  electricity  out  of  the 
cloud  silently  and  gradually,  and  in  this  way  the  disaster 
may  be  prevented. 

Who  first  took  lightning  from  the  clouds  ?— Dr. 
Franklin  first  drew  electricity  from  the  clouds  in  such  a 
way  as  to  be  able  to  examine  it,  and  prove  that  lightning 
is  nothing  but  electricity. 

How  did  he  doit?— This  discovery  of  the  nature  of 
lightning  was  one  of  the  most  important  ever  made  in 
the  science,  and  yet,  Dr.  Franklin  made  it  simply  byj^y- 
ing  a  kite  in  a  thunder-shower  (Fig.  82). 

He  made  his  kite  of  silk  instead  of  paper,  and  sent  it  up 
with  a  hempen  cord  ending  in  a  piece  of  silk  cord,  by 
which  the  kite  was  held.  It  is  said  that  he  fastened  a  door- 
key  to  the  lower  end  of  the  hempen  cord,  and  that  after 
his  kite  had  been  for  some  time  sailing  among  the  clouds 
he  touched  this  key  with  his  knuckle  and  drew  a  spark 
of  electricity  from  it.  The  electricity  in  the  cloud  entered 
the  kite,  and  came  down  the  hempen  string  to  the  key, 
but  could  not  go  any  farther  because  t"  e  silk  cord  was  a 
non-conductor.  When  the  doctor  presented  his  hand  the 
electricity  in  the  key  leaped  into  his  knuckle. 

Can  electricity  act  through  no --conductors  ? — 

The  following  experiment  will  show  that  electricity  can 
and  will  act  through  non-conductors. 

"We  will  suppose  the  ball  C  (Fig.  83)  to  be  insulated 
and  charged  with  electricity.  Another  insulated  conduc- 
tor, AB.  is  slowly  moved  toward  the  ball,  and  when  the 


ELECTRICITY  BY  FRICTION. 

Fig.  82. 


160  NATURAL  PHILOSOPHY. 

end  of  it  is  still  at  some  distance,  the  pith-balls,  which  have 
all  the  time  hung  vertically,  suddenly  jump  away  and 
remain  as  shown  in  the  picture. 


Fig.  S3. 


The  action  of  the  pith-balls  shows  that  the  insulated 
conductor  AB  is  electrified,  and  we  see  that  the  electricity 
of  the  ball  C  must  have  acted  through  the  air.  It  would 
have  the  same  effect  through  glass  or  other  non-con- 
ductors. 

What  is  this  action  called  ?— This  action  of  a 
charged  body  through  non-conductors  is  called  induction. 

What  is  its  effect  ? — It  will  be  found  that  the  insu- 
lated body  AB  is  not  electrified  all  over  its  surface  alike. 
Both  kinds  of  electricity  are  found  upon  it.  The  ends 
are  most  powerfully  electrified,  and  the  two  ends  are  in 
opposite  conditions.  The  end  B,  most  distant,  has  the 
same  kind  of  electricity  as  the  charged  ball  C. 

Such  will  always  prove  to  be  the  case.  Induction  al- 
ways causes  ~both  kinds  of  electricity  to  appear  on  the 
surface  of  an  insulated  body. 

How  do  we  describe  this  condition  of  an  insu- 


ELECTRICITY  BY  FRICTION.  IQ[ 

lated  body? — When  botli  kinds  of  electricity  are  devel- 
oped on  the  surface  of  a  body,  it  is  said  to  be  polarized. 

Describe  the  electrical  bells. — Fig.  84  shows  a 
chime  of  bells,  which  are  to  be  rung  by  electricity.  Notice 
how  they  are  arranged.  The  two  Fig.  84 

outside  bells  are  fastened  by 
metal  chains  to  a  rod  of  metal 
which  hangs  from  the  end  of  the 
prime  conductor  of  an  electrical 
machine.  The  middle  bell  is 
hung  by  a  silk  thread  and  has  a 
chain  passing  from  it  to  the  floor. 
Finally,  notice  two  little  balls  of 
metal  between  the  bells :  these 
balls  are  hung  by  silk  threads 
also.  When  the  machine  is  in 

operation,  these  little  balls  will  fly  back   and  forth    and 
ring  the  bells  loudly. 

Explain  this  experiment. — The  electricity  from  the 
machine  passes  down  the  chains  into  the  outside  bells,  but 
it  can  not  get  into  the  middle  bell  nor  into  the  little  balls, 
because  their  silk  cords  are  not  conductors.  Now  when 
one  outside  bell  is  charged,  its  electricity  will  act  through 
the  air  upon  the  little  ball  and  polarize  it,  and  the  elec- 
tricity on  that  side  of  the  ball  nearest  the  bell  is  the  other 
kind  from  that  in  the  bell  itself.  Then  the  two  unlike 
kinds  attract  each  other,  and  this  is  what  brings  the  ball 
to  strike  the  bell. 

But  when  the  ball  touches  the  bell  it  takes  electricity 
from  it,  and  then  the  two,  having  the  same  kind  of  elec- 
tricity, repel  each  other.  This  is  what  throws  the  little 
ball  against  the  middle  bell.  But  when  it  touches  the 
middle  bell  it  gives  away  its  electricity  and  is  ready  to  be 


102  NATURAL  PHILOSOPHY.- 

polarized  over  again.  It  is  first  polarized,  then  attract-  d, 
then  i'1'pcllt-d  over  and  over  again,  perhaps  fitly  times 
while  we  are  giving  the  explanation  once. 

The  chain  from  the  middle  bell  is  to  conduct  away  the 
electricity  brought  by  the  little  balls.  This  is  needed  to 
keep  the  middle  bell  from  becoming  charged,  and  stopping 
the  operation. 

Describe  the  dancing  pith-balls. — If  a  plate  of 
metal  is  hung  from  the  machine  just  above  another  simi- 
lar plate  which  lies  upon  the  table,  and  then  if  a  handful 
of  pith-balls  is  just  between  the  two,  these  balls  will  per- 
form a  lively  dance  whenever  the  machine  is  put  in  opera- 
tion. Sometimes  a  glass  shade  is  put  over  the  disks  to 
keep  the  balls  from  Hying  away  from  between  them,  as 
they  are  otherwise  very  sure  to  do. 

Each  little  ball  is  first  polarized  by  the  electricity  in 
the  upper  plate,  and  then  attracted  and  afterward  re- 
pelled. 

What  do  these  experiments  illustrate? — These 
experiments  illustrate  the  fact  that  no  light  body  is  ever 
attracted  until  after  it  is  polarized  by  the  charged  body 
toward  which  it  flies. 

Describe  the  charged  goblet  of  water. — A  long 
time  ago  a  gentleman  in  France  was  experimenting  with 
electricity  to  see  how  it  would  affect  water.  A  chain 
from  the  conductor  of  his  machine  hung  down  into  a  gob- 
let of  the  liquid,  but  it  seemed  to  produce  no  effect,- and 
he  was  about  to  take  it  away.  He  seized  the  goblet  in 
one  hand  and  took  hold  of  the  chain  with  the  other.  The 
moment  that  his  fingers  touched  the  chain,  he  received  a 
shock  which  convulsed  his  hands  and  gave  him  such  a 
fright  that  he  did  not  quite  get  over  it  in  two  days. 


ELECTRICITY  BY  FRICTION.  163 

Explain  this  experiment. — The  water  in  the  goblet 
was  charged  with  electricity  1'rom  the  machine,  and  when 
the  hand  was  placed  around  the  outside,  it  and  the  glass 
were  polarized,  so  that  the  outside  and  inside  of  the  goblet 
were  in  opposite  conditions.  When  the  other  hand 
touched  the  chain  the  arms  and  body  made  a  conducting 
road  through  which  the  two  electricities  could  get  together, 
and  their  action  through  the  person  caused  the  curious 
and  unexpected  feeling  called  the  shock. 

What  should  we  notice  in  the  arrangement  of 
the  goblet  ? — Now  we  notice  that  there  were  three 
things — the  water  inside,  the  hand  outside,  and  the  glass 
letw<.  en,  arid  that  the  water  and  the  hand  are  good  con- 
ductors, while  the  glass  is  not  To  put  it  in  few  words, 
we  see  that  there  were  two  conductors  kept  apart  by  a  non- 
conductor. 

Describe  the  Leyden  jar.— Any  two  conducting  sub- 
stances kept  apart  by  glass  will  answer  just  as  well,  or 
indeed  very  much  better  than  the  water  and  the  hand. 
Tinfoil  is  pasted  over  the  surfaces  of  a  glass  jar,  both 
inside  and  outside,  to  within  a  few  inches  of  the  top,  and 
then  the  jar  is  covered  with  a  cover  made  of  hard  wood, 
through  which  passes  a  brass  rod.  There  is  a  ball  on  the 
top  of  this  rod,  and  a  chain  at  its  lower  end  that  reaches 
down  to  the  bottom  of  the  jar. 

How  may  the  jar  be  charged? — Let  the  jar  be  held 
in  the  hand  with  its  knob  very  near  to  the  conductor  of 
the  machine.  Sparks  of  electricity  will  fly  into  the  knob; 
in  a  dark  room  they  look  like  little  flashes  of  lightning. 
After  a  while,  when  the  sparks  are  .feeble,  the  jar  is  said 
to  be  charged. 

How  may  it  be  discharged  ?— Should  a  person  acci- 
dentally touch  the  knob  of  the  charged  jar  with  one  hand 


1G4  NATURAL  PHILOSOPHY. 

and  the  outside  tinfoil  with  the  other,  he  would  feel  a 
shock  which  would  startle  and  perhaps  injure  him.  His 
body  is  a  good  conductor,  and  the  shock  is  due  to  the 
discharge  of  the  jar  through  it.  A  discharge  will  always 
occur  when  any  conductor  reaches  from  the  knob  to  the 
outside  coating  of  the  jar.  A  bent  wire  with  a  glass 
handle  is  generally  used  for  the  purpose. 


ELECTRICITY  BT  CHEMICAL  ACTION. 


What  is  meant  by  chemical  action  ? — "When  a 
piece  of  paper  burns  it  ceases  to  be  paper,  as  every  one 
knows,  and  changes  into  smoke  and  ashes.  The  nature 
of  the  substance  is  changed  during  the  action.  Now  this 
shows  what  we  mean  by  chemical  action.  It  is  an  action 
by  which  the  nature  of  a  substance  in  changed. 

We  will  mention  another  case.  Put  some  bits  of  zinc 
into  a  goblet  and  pour  upon  them  some  weak  sulphuric 
acid.  The  fluid  will  soon  begin  to  boil  violently,  and 
bubbles  of  gas  will  be  given  off,  so  that  often,  if  a  lighted 
match  is  held  near,  the  gas  will  take  fire.  This  will  give 
the  curious  appearance  of  water  on  fire.  After  a  while 
the  action  will  stop,  but  not  until  much  and  perhaps  all 
the  zinc  has  been  used  up. 

Why  is  this  action  a  chemical  action? — In  this 
case  both  the  zinc  and  the  acid  are  changed  into  other 
substances,  and  on  this  account  the  action  is  called  a 
chemical  action. 

Will  it  produce  electricity  ? — Now  let  a  strip  of 
zinc  and  another  of  copper  be  placed  side  by  side  in  a  glass 
vessel  nearly  full  of  weak  sulphuric  acid,  but  do  not  let 
them  touch  each  other.  Have  a  wire  fastened  to  the 
upper  end  of  each  metal.  It  will  be  found  that  whenever 
th  se  wires  come  together  electricity  will  act  through 


166  NATURAL   PHILOSOPHY. 

them.  This  electricity  is  due  to  the  chemical  action 
going  on  in  the  vessel. 

What  is  this  apparatus  called  ? — This  simple  appa- 
ratus is  called  a  Voltaic  circuit.  The  electricity  it  gives 
is  ol'ten  called  Galvanism  and  often  Voltaic  electricity. 
These  names  were  given  in  honor  of  Galvani,  who  first 
studied  this  force,  and  of  Yolta,  who  also  made  ii  a  study, 
and  found  out  many  new  things  about  it. 

What  are  the  poles  of  the  circuit?— The  ends  of 
the  wires  are  commonly  called  the  poles,  of  the  circuit. 
One  is  called  the  positive  pole  and  the  other  is  called  the 
negative  pole. 

When  the  poles  are  in  contact,  or  when  there  is  any 
conductor  which  joins  them  toge.ther,  the  circuit  is  said 
to  be  closed,  but  when  they  are  separated  the  circuit  is 
said  to  be  open. 

What  effects  can  this  circuit  produce  ? — The  elec- 
tric force  in  this  simple  circuit  is  very  weak.  It  can  make 
its  presence  known  by  a  feeble  spark  seen  in  the  dark  at 
the  moment  when  the  wires  are  separated,  but  which  can 
be  seldom  seen  at  all  in  daylight.  It  also  causes  bubbles 
of  gas  to  rise  alongside  of  the  copper  plate  when  the  circuit 
is  closed.  And,  what  is  more  curious  still  is  that  it  will 
turn  a  magnetic  needle,  when  the  wires  are  laid  length- 
wise of  the  needle,  without  touching  it.  You  will  learn 
more  of  these  effects  at  some  future  time. 

Can  greater  effects  be  obtained? — Very  much 
greater  effects  can  be  obtained  by  using  a  different  appa- 
ratus. One  of  the  best  kinds  is  the  Bunseris  lattery. 

The  picture  Fig.  85  will  help  us  to  understand  how 
the  Bunsen's  battery  is  arranged.  It  consists  of — 

1.  A  glass  or  earthenware  vessel,  containing 

2.  Dilute  sulphuric  acid.     In  this  stands 


ELECTRICITY  BY  CHKMICAL  ACTION. 

3.  A  hollow  cylinder  of  zinc.     Inside  of  this  is 

4.  A  porous  earthen  cup,  filled  with 

5.  Strong  nitric  acid.     And  finally  in  this 

6.  A  rod  or  block  of  carbon  is  immersed. 

Fig.  So. 


167 


One  wire  or  metallic  bar  goes  from  the  carbon  ;  this  is 
the  positive  pole:  another  goes  from  the  zinc,  and  this  is 
the  negative  pole. 

How  may  the  power  of  the  battery  be  increas- 
ed?—By  joining  several  of  these  single  batteries  together 
electricity  of  almost  any  power  may  be  obtained. 

The  zinc  of  each  cell  may  be  joined  to  the  carbon  of 
the  next.  The  first  carbon  is  the  positive  pole,  and  the 
last  zinc  is  the  negative  pole.  When  these  poles  are 
joined  together  a  powerful  electric  force  is  obtained,  by 
which  most  wonderful  effects  are  caused. 


168 


NATURAL  PHILOSOPHY. 


Describe  the  electric  heat.— If  a  fine  iron  wire  is 
stretched  between  the  poles  of  a  strong  battery  it  will  be 
quickly  heated  white  hot,  and  actually  burned  up,  even 
when  several  inches  long.  Copper,  zinc,  and  other  metals 
melt  readily  and  are  burned  by  electricity. 

How  is  the  electric  light  obtained  ? — In  the  pic- 


tare,  Fig.  80,  the  electric  light  is  represented  as  in  use. 
The  battery,  a  very  strong  one  of  50  or  perhaps  100  cells, 


ELECTRICITY  BY  CHEMICAL  ACTION.  169 

stands  upon  the  floor.  The  lamp  stands  upon  the  table. 
The  light  is  obtained  by  having  the  poles  tipped  with 
charcoal  and  then  drawing  them  a  little  way  apart.  It 
is  of  the  most  dazzling  brightness.  The  light  is  not 
due  to  the  burning  of  the  charcoal,  for  it  is  as  bright 
when  made  in  a  vacuum  where  the  carbon  cannot  burn 
at  all. 

In  the  picture  this  light  is  being  used  in  a  microscope. 
It  is  made  to  pass  through  an  insect  and  afterwards 
through  convex  lenses,  which  form  a  magnified  image  of 
the  creature  upon  the  screen. 

"With  this  instrument  it  is  possible  to  show  the  smallest 
objects  magnified  almost  indefinitely.  A  human  hair 
will  appear  as  large  as  a  broomstick ;  an  ordinary  flea 
will  look  the  size  of  a  sheep,  and  the  smallest  animalcules 
will  be  visible  in  all  their  beauty  of  form  and  color  as 
clearly  as  if  they  were  seen  with  the  naked  eye." 

The  electric  light  has  been  used  also,  sometimes,  to  en- 
able workmen  to  labor  at  night.  When  it  is  necessary  to 
accomplish  a  great  work  speedily,  it  may  go  on  without 
stopping,  the  sun  giving  the  workmen  light  by  day  and 
the  electric  light  by  night.  See  Fig.  87. 

How  may  water  be  decomposed  ? — We  need  only 
put  the  poles  of  a  strong  battery  into  water  and  bubbles 
of  gas  wiil  rapidly  rise  from  both.  The  water  is  changed 
into  two  gases. 

The  picture,  Fig.  88,  will  show  us  how  the  experiment 
is  usually  made. 

We  see  that  the  wires  from  a  battery  go  up  into  a 
vessel  of  water  and  that  two  tall  tubes  are  placed  over 
them.  These  tubes  are  there  to  catch  the  little  bubbles 
of  gas  into  which  the  water  is  changed.  They  were  filled 
with  water  at  the  beginning  of  the  experiment,  but  as  the 


170 


NATURAL  PHILOSOPHY. 

Fi?.  87. 


ELECTRICITY  BY  CHEMICAL  ACTION. 


171 


bubbles  rise  they  drive  the  water  out.     One  tube  is  tilling 
twice  as  fast  as  the  other,  you  may  notice. 


Fig.  SS. 


What  gases  are  these  ?— One  of  these  gases  is  hydro- 
gen, the  other  is  oxygen.  The  hydrogen  is  given  off  most 
rapidly. 

The  electricity  in  this  case  helps-  us  to  learn  that  water 
is  made  up  of  two  very  light  and  colorless  gases,  hydrogen 
and  oxygen,  and  that  it  contains  twice  as  much  of  the 
first  a^  of  the  second. 

A  great  many  other  substances  may  be  decomposed  by 
electricity. 


ELECTRO-MAGNETISM. 


What  apparatus  needed  to  show  the  effect  of 
electricity  upon  iron  ?— Take  a  long  piece  of  covered 
copper  wire  and  wind  it  many  times  around  a  rod  of  soft 
iron.  Fix  one  end  of  this  wire  to  one  pole  of  the  battery 
and  the  other  end  to  the  other  pole.  The  electricity  will 
then  act  around  the  iron  rod  and  we  can  study  its  effects. 

What  effect  is  produced  ? — Open  the  circuit  by  tak- 
ing the  wire  away  from  one  pole  of  the  battery,  and  we 
may  by  trial  find  that  the  iron  has  no  especial  attraction 
for  small  bits  of  iron  which  are  brought  in  contact  with 
it ;  but  close  the  circuit,  and  instantly  the  bits  of  iron  will 
cling  to  the  rod  and  be  held  by  it  as  long  as  the  elec- 
tricity acts. 

This  experiment  shows  that  electricity  makes  a  bar  of 
iron  magnetic  by  acting  around  it. 

What  are  such  magnets  called  ? — Such  magnets  are 
called  electro-magnets  because  their  magnetism  is  caused 
by  electricity. 

In  what  form  are  they  usually  made?— These 
electro-magnets  are  generally  made  in  the  shape  of  the 
horse-shoe  magnet,  and  the  wire  is  wound  a  great  many 
times  around  each  branch. 

In  the  picture  (Fig.  89)  we  see  one  of  the  electro-mag- 
nets fastened  in  a  frame. 


ELECTRO-MAGNETISM. 


173 


This  picture  also  shows  the  result  of  a  curious  experi- 
ment. A  box  of  nails  is  shown  'below.  This  box  was 
lifted  until  the  magnet  touched  the  nails  and  then  was 

FiR.  89. 


slowly  let  down  again.  The  magnet  lifted  all  the  nails  it 
touched :  these  nails  lifted  others,  and  others  then  clung 
to  them  until,  as  you  see  in  the  picture,  a  chain  of  nails 
hung  from  the  ends  of  the  magnet  and  rested  upon  the 
box  below. 

How  long  will  the  iron  stay  magnetic  ? — Just  at 
the  moment  the  electric  circuit  is  opened  the  nails  will 
drop,  every  one  into  the  box.  The  iron  will  be  a  magnet 
only  while  the  electricity  is  acting  around  it. 

What  instrument  acts  on  this  principle  ?— The 


174  NATURAL  PHILOSOPHY. 

electric  telegraph  acts  upon  tins  principle.  In  one  city 
there  is  a  "  key  "  by  which  a  person  may  open  and  close 
an  electric  circuit  as  often  as  he  pleases.  The  wires  of 
this  circuit  reach  over  the  country  to  a  distant  city  an  I 
are  there  joined  to  the  coils  of  an  electro-magnet.  Just 
above  the  poles  of  this  magnet  is  an  armature  kept  a  little 
way  from  them  by  a  spring. 

Now  let  a  person  press  the  key  with  his  finger  and  close 
the  circuit :  the  electricity  will  dart  through  the  wires  to 
the  distant  city  and  around  the  electro-magnet,  and  the 
magnet  will  pull  the  armature  down.  When  he  lifts  his 
finger  the  electricity  will  not  act;  the  magnet  ceases  to 
be  magnetic  and  the  armature  is  lifted  by  the  spring. 
Just  as  often  as  he  presses  the  key  the  armature  will  be 
drawn  down. 

The  next  thing  to  know  in  order  to  understand  this 
wonderful  instrument,  is  that  there  is  a  steel  point  fastened 
to  the  armature,  so  that  every  time  the  armature  is  drawn 
to  the  magnet,  the  point  is  pulled  against  a  strip  of 
paper  and  makes  a  mark  upon  it.  A  person  in  one  city 
can  thus  be  making  marks  upon  paper  in  another  city 
many  miles  away. 

The  marks  consist  of  dots  and  straight  lines,  and  each 
letter  of  the  alphabet  is  represented  by  some  arrangement 
of  these  marks.  For  instance,  a  dot  followed  by  a  dash, 

thus : ,  means  A ;  and  a  dash  followed  by  a  dot,  thus, 

-  -  stands  for  N  ;  while  a  dash  and  two  dots stands 

for  D.  You  see,  then,  how  the  man  at  the  key  may  write 
the  word  AND  by  making  these  dots  and  dashes.  It 
would  look  like  this,  -  -  -  -  -,  and  one  who  knows  the 

alphabet  by  dots  and  dashes  can  read  these  characters 
even  when  a  thousand  miles  away  from  the  place  from 
which  the  message  is  sent. 


ELECTRO-MAGNETISM.  175 

This  description  is  only  an  outline  of  the  plan  of  the 
electric  telegraph.  Every  boy  and  girl  should  seek  a 
chance  to  see  the  instrument  itself,  for  from  it  can  be 
learned,  better  than  from  any  book,  just  how  the  messages 
are  sent  and  taken. 


MACHINERY. 


What  is  shown  in  Fig.  90  ?— In  Fig.  90  we  see  the 
picture  of  a  man  trying  to  move  a  block  of  stone.  It  is. 
much  too  heavy  for  him  to  lift,  and  you  notice  that  he  has 
taken  a  long  bar  to  assist  him.  Putting  one  end  of  it 
under  the  stone,  and  resting  the  bar  upon  a  block  C,  he 
pushes  down  upon  the  other  end,  and  in  this  way  lifts  the 
stone. 

Fig,  90. 


Explain  the  action  more  fully. — We  see  that  the 
bar  rests  upon  the  prop  C,  and  that  the  end  B  cannot  be 
pushed  down  without  moving  the  other  end  A,  up.  Now 
when  the  part  C  B  is  so  much  longer  than  the  part  C  A, 
the  man  by  his  own  weight  can  lift  a  stone  very  much 
heavier  than  he  himself  is. 

What  is  this  bar  called  ?— This  bar  is  called  a  lever. 
This  name  is  given  to  any  bar  that  can  be  used  in  this 
manner.  A  lever  is  an  inflexible  bar  which  can  turn 
freely  upon  a  pivot  or  prop. 

Define  Power,  Weight,  and  Fulcrum. — The  strength 
which  the  man  exerts  at  B  is  called  the  power/  we  speak 


MACHINERY.  177 

of  the  stone  to  be  moved  as  the  weight,  and  call  the  prop 
C  the  fulcrum.. 

Levers  are  not  always  moved  by  hand,  nor  are  they 
always  used  to  move  stones,  and  yet  these  same  terms  are 
used.  The  power  is  any  force  by  which  the  lever  is  to  be 
moved.  The  weight  is  any  resistance  which  is  to  be  over- 
come. The  fulcrum  is  the  support  on  which  the  lever 
moves. 

What  does  Fig.  91  A  show  ?— In  Fig.  91  A,  the  lever 
is  shown  without  the  laborer  and  the  stone.  P  represents 
the  place  of  the  power ;  "VV  represents  the  place  of  the 
weight,  and  F  the  fulcrum.  The  fulcrum  is  in  this  case 
between  the  power  and  the  weight. 

Fig.  91. 


II 


Is  this  always  the  case  ? — But  this  is  not  always  the 
order  of  arrangement.  In  Fig.  91  B,  the  weight  W  is  be- 
tween the  fulcrum  F  and  the  power  P.  The  weight  being 
near  the  fulcrum,  is  lifted  by  raising  the  more  distant  end 
of  the  lever. 

In  Fig.  91  C.  the  power  is  between  the  fulcrum  and  the 
weight.  Here  also  the  power  must  pull  upward  to  lift  the 
weight.  Now  these  three  figures  represent  levers  of  the 
three  classes. 

Describe  the  three  classes  of  lever. — In  a  lever  of 


178  NATURAL  PHILOSOPHY. 

the  first  class  the  fulcrum  is  between  the  power  and  the 
weight  (Fig.  91  A).  In  a  lever  of  the  second  class  the 
weight  is  between  the  power  and  the  fulcrum  (Fig.  91  B). 
In  a  lever  of  the  third  class  the  power  is  between  the  ful- 
crum and  the  weight  (Fig.  91  C). 

All  levers,  of  whatever  form  or  use,  belong  to  these 
three  classes. 

Mention  some  levers  of  the  first  class. — The  handle 
of  a  common  pump  is  a  lever  of  the  first  class :  the  piston 
and  the  water  are  the  weight,  the  hand  of  whomsoever  does 
the  work  is  the  power,  while  the  pivot  on  which  the  handle 
turns  is  the  fulcrum. 

The  balance  of  a  .tradesman  is  another  example :  the 
body  to  be  weighed,  put  into  one  scale-pan,  is  the  weight ; 
the  weights  put  into  the  other  pan  are  the  power ;  while 
the  pivot  on  which  the  beam  turns  is  the  fulcrum. 

Examples  of  the  second  class. — The  handles  of  a 
whealbarrow  are  levers  of  the  second  class:  the  axle  of 
the  wheel  is  the  fulcrum  on  which  they  turn  when  lifted  ; 
whatever  is  placed  in  the  barrow  is  the  weight,  while  the 
hand  of  the  laborer  is  the  power  to  lift  it. 

An  oar  is  another  example.  The  boat  is  the  weight  to 
be  moved ;  the  hand  of  the  boatman  is  the  power  to  move 
it ;  while  the  water  against  which  the  other  end  of  the  oar 
presses  is  the  fulcrum. 

Example  of  the  third  class. — When  a  ladder  is  raised 
by  resting  one  end  on  the  ground  and  lifting  upon  a  round 
somewhat  farther  up,  it  is  a  lever  of  the  third  class.  The 
end  on  the  ground  is  the  fulcrum;  the  hand  of  the  man  is 
the  power ;  while  the  weight  of  the  ladder,  most  of  which 
is  beyond  the  hand,  is  the  weight. 

What  relation  exists  between  power  and  weight  ? 
— In  Fig.  90,  if  the  distance  from  A,  the  place  where  the 
weight  rests  on  the  lever,  to  C,  the  fulcrum,  is  one-fourth 
the  distance  from  C  to  B,  then  the  weight  may  be  balanced 


MACHINERY.  179 

by  a  power  only  one-fourth  as  great.  The  power  will  be 
j.ust  as  many  times  less  than  the  weight  as  the  distance 
from  it  to  the  fulcrum  is  times  greater  than  the  distance 
from  the  weight  to  the  fulcrum.  If  the  distance  of  the 
power  from  the  fulcrum  is  ten  times  as  far  as  the  distance 
from  the  weight  to  the  fulcrum,  then  the  weight  will  be 
balanced  by  a  power  only  one-tenth  as  great  as  itself. 
f>  State  the  law. — This  principle,  briefly  stated,  is  as  fol- 
lows :  "  The  power  and  weight  will  balance  each,  other 
when  they  are  to  each  other  inversely  as  their  distances 
from  the  fulcrum." 

This  principle  is  called  the  law  of  equilibrium  for  the 
lever.  It  holds  good  in  all  the  three  classes. 

To  move  the  weight,  the  power  must  be  a  little  greater 
than  this  principle  would  make  it. 

What  is  a  compound  lever  ? — When  two  or  more 
levers  are  made  to  act  one  upon  another  in  succession,  so 
that  a  power  applied  to  the  first  lifts  a  weight  applied  to 
the  last,  the  instrument  is  called  a  compound  lever. 

What  does  Fig-.  92  show? — Fig.  92  shows  how  a 
weight  may  be  lifted  by  fastening  one  end  of  a  rope  to  it 

Fig.  92. 

c 


and  winding  the  rope  up  on  a  cylinder.     In  this  way  water 
is  often  raised  from  deep  wells.     By  turning  the  crank 


180  NATUKAL  PHILOSOPHY. 

the  upper  part  of  the  rope  is  wound  upon  the  cylinder,  and 
the  bucket,  hooked  upon  the  lower  end,  is  raised. 

What  often  takes  the  place  of  the  crank? — Instead 
of  a  crank  13,  to  turn  the  cylinder,  a  wheel  C  is  very  often 
used.  The  power  is  applied  to  the  circumference  of  the 
/heel,  sometimes  by  means  of  a  rope,  sometimes  by  means 
•i  a  band,  sometimes  by  means  of  cogs,  and  in  various 
other  ways. 

What  are  such  machines  called? — A  machine  such 
as  represented  in  Fig.  92  is  called  a  "  Wheel  and  axle.'1'' 
The  cylinder  upon  which  the  rope  winds  is  the  "<:«?&>," 
while  the  "wheel"  may  be  an  actual  wheel  or  a  crank, 
both  of  which  are  shown  in  the  picture,  or  it  may  have 
other  forms  still.  Whatever  shape  this  part  may  have 
the  machine  is  called  the  "  wheel  and  axle." 

What  is  the  relation  between  the  power  and  the 
•weight? — When  the  weight  is  just  as  many  times  greater 
than  the  power  as  the  radius  of  the  wheel  is  greater  than 
the  radius  of  the  axle,  the  two  forces  will  just  balance. 
In  other  words :  "  The  power  and  weight  will  balance, 
when  the  power  is  to  the  weight  as  the  radius  of  the  axle 
is  to  the  radius  of  the  wheel." 

This  principle  is  called  the  law  of  equilibrium  for  the 
wheel  and  axle.  To  move  the  weight,  the  power  must  be 
made  greater  than  this  law  requires. 

How  are  wheels  and  axles  often  combined  ? — Many 
wheels  and  axles  are  sometimes  turned  by  a  single  power. 
In  this  case  motion  is  communicated  from  wheel  to  axle 
or  from  Tixle  to  wheel  by  means  of  bands  or  cogs.  If  by 
u  power  on  one  wheel  its  axle  is  turned,  and  a  band  passes 
around  this  axle  and  a  second  wheel,  the  second  axle  will 
be  turned  also.  Great  power  may  in  this  way  be  ob- 
tained. 

How  may  rapid  motion  be  secured? — If  the  power 
be  applied  to  the  circumference  of  the  axle  instead  of 


MACHINERY. 


181 


Fig.  93. 


the  wheel,  and  if  a  band  pass  around  the  wheel  and  a  second 
axle,  the  second  wheel  will  be  put  into  very  rapid  motion. 

What  is  shown  in  Fig.  93  ? —  In  this  figure  we  can 
see  how  a  heavy  weight  may  be 
lifted  by  fastening  it  to  the  end 
of  a  rope  which  passes  up  over  a 
grooved  wheel,  and  then  pulling 
downward  upon  the  other  end. 

What  is  such  a  grooved 
•wheel  called? — A  grooved  wheel 
used  for  such  a  purpose  is  called 
a  pulley  ;  and  in  this  case,  since  it 
is  firmly  fastened  in  a  fixed  sup- 
port, it  is  called  a  fixed  pulley. 

What  advantage  in  its  use? — The  only  advantage 
gained  by  means  of  the  fixed  pulley  consists  in  being  able 
to  change  the  direction  in  which  the  power  acts.  A  man, 
for  example,  can  exert  his  strength  to  better  advantage 
putting  downward  than  lifting  upward,  and  if  a  load  is 
to  be  lifted  the  fixed  pulley  allows  him  to  use  his  power  in 
this  better  way.  He  gains  in  no  other 
way ;  if  the  load  weighs  100  Ibs.,  he  must 
pull  Avith  a  force  equal  to  100  Ibs.,  and  in- 
deed a  little  more,  since  the  rope  and  pulley 
take  up  some  of  his  strength  to  move  them. 
What  is  a  movable  pulley? — The  case 
is  very  different  when  the  pulley  is  ar- 
ranged as  in  Fig.  94.  In  this  arrangement 
the  weight  is  hung  from  the  axis  of  the  pul- 
ley, B,  and  is  to  be  lifted  by  means  of  the 
rope  which  is  fastened  to  the  beam  at  A, 
and  then  after  passing  under  the  pulley  13, 
goes  over  the  fixed  pulley  C.  By  pulling  upon  the  rope 
at  P,  the  pulley  B  will  be  lifted  and  will  carry  the  weight 


Fig.  94. 


182 


NATURAL  PHILOSOPHY. 


up  with   it.     A  pulley  which  moves  with  the  weight  is 
called  a  movable  pulley. 

"What  advantage  is  gained? — Now  it  is  easy  to  see 
that  the  weight  is  held  up  by  the  two  branches  of  rope,  m 
and  n,  and  that  each  branch  holds  one-half  of  it.  But  the 
half  which  rests  on  m  is  sustained  by  the  beam,  leaving 
only  the  other  half,  which  rests  on  n,  to  be  lifted  by  the 
power  at  P. 

Fig.  95.  When,  with  a  movable  pulley,  there  are 

two  branches  of  rope  to  sustain  the  weight, 
the  power  ma}'  be  only  one-half  the  weight. 
In  Fig.  95  there  are  three  branches  of 
the  rope  which  hold  the  weight  and  share 
it  equally  between  them.  In  this  case  the 
power  need  be  only  one-third  as  great  as 
the  weight  to  balance  it. 

Wnat  general  principle  does  this  il- 
lustrate ? — In  all  cases  the  power  needed 
to  balance   any  weight  wrill   be  found   by 
dividing   that   weight   by   the   number   of 
branches  of  the  rope  which  supports  it. 

Will  this  law  apply  in  all  cases  ? — In  the  cases  con- 
sidered you  will  notice  that  there  is  a  single  rope  winding 
around  all  the  pulleys.  Now  the  law  holds  good  when- 
ever the  weight  is  supported  in  this  way,  provided  the 
branches  of  the  rope  are  parallel.  .There  are  a  great  many 
other  ways  of  arranging  the  pulleys,  not  as  common  as 
this,  however,  and  in  such  cases  the  law  stated  above  does 
not  hold  good. 

Mention  some  purposes  for  -which  pulleys  are 
used. — Pulleys  are  often  used  for  lifting  heavy  articles  of 
merchandise  to  the  upper  stories  of  warehouses.  They 
may  be  seen  also  where  buildings  of  stone  are  being  erect- 
ed, and  heavy  blocks  are  to  be  raised  to  considerable 
height.  But  more  numerous  than  anywhere  else,  you  will 


MACHINERY.  183 

find  pulleys  on  shipboard,  where  they  are  used  by  the 
seamen  in  managing  the  rigging  of  the  ship. 

What  is  an  inclined  plane  ? — "When  a  drayman  wishes 
to  lift  a  cask  of  sugar  from  the  sidewalk  to  his  dray  he 
does  not  lay  hold  of  it  and  raise  it  vertically,  as  he  migh* 
do  with  a  lesser  weight,  but  he  accomplishes  the  work 
far  more  easily  by  rolling  it  up  along  a  plank  reaching 
obliquely  from  the  ground  to  the  dray.  The  inclined 
surface  of  the  plank  is  called  an  inclined  plane.  Any 
inclined  surface  over  which  weights  are  to  be  moved  is 
an  inclined  plane. 

What  is  meant  "by  the  terms  length  and  height  of 
the  plane  ? — In  Fig.  96  a  weight  W  is  shown  resting  on 
an  inclined  surface  A  B,  balanced  by  a  smaller  weight  P. 
Kow  the  distance  A  B  is  called  the  length  of  the  inclined 
plane,  and  the  vertical  distance  C  B  is  called  the  height  of 
the  plane. 

Fig.  96. 


In  the  plane  used  by  the  drayman  the  length  of  the 
plank  from  the  sidewalk  to  the  dray  is  the  length  of  the 
inclined  plane,  while  the  height  of  the  dray  above  the 
walk  is  the  height  of  the  plane. 

What  relation  exists  between  power  and  weight  ? 
— If  the  weight  W  is  100  Ibs.  and  the  height  of  the  plane 
is  one  half  the  length,  then  the  power  to  balance  the 
weight  need  be  only  one-half  the  weight,  or  50  Ibs. 

If  the  height  is  ^  Of  the  length  of  the  plane,  the  power 
need  be  only  y1^  the  weight. 

The  general  principle  or  law  is  this ;  the  power  and 


184 


NATURAL  PHILOSOPHY. 


weight  will  balance  when  the  power  is  to  the  weight  as 
the  height  of  the  plane  is  to  its  length. 

Under  wfrat  conditions  will  this  law  hold  good  ? — 
This  law  holds  good  only  when  the  power  is  exerted  in  a 
direction  parallel  to  the  length  of  the  plane;  in  any  other 
direction  the  relation  is  different.  Moreover,  friction  of 
the  weight  upon  the  plane  has  much  to  do  with  the  re- 
lation of  power  to  weight.  The  law  would  be  quite  true 
only  when  friction  did  not  exist,  a  case  which  never  occurs 
in  practice.  If  the  weight  is  to  be  balanced  on  the  plane, 
then  friction  helps  the  power  to  do  it ;  but  if  the  weight  is 
to  be  moved  up  the  plane,  friction  is  a  hindrance  instead 
of  a  help. 

Explain  Fig.  97. — This  picture,  Fig.  97,  shows  how 
barrels  are  drawn  up  from  or  let  down  into  a  cellar.  It  is 
a  case  of  the  inclined  plane  which  you  can  easily  under- 
stand and  explain  without  further  help. 

Fig.  97. 


How  does  a  woodman  sometimes  split  his  blocks  ? 
— When  blocks  of  wood  are  to  be  split,  a  smaller  block  of 
wood  or  metal,  made  thick  at  one  end  and  tapering  to  an 


MACHINERY.  185 

edge  at  the  other,  is  driven  into  the  end  of  it,  as  shown  in 
Fig.  98.     The  tapering  block  is  called  a  wedge. 


Fig.  98. 


What  are  the  power  and  weight  in  this  case  ? — 

The  energy  of  the  woodman's  blows  upon  the  back  of  the 
wedge  is  the  power  /  the  cohesion  of  the  wood  is  the 
weight,  and  it  is  not  easy  to  find  the  relation  between  these 
two  forces.  Hence  we  cannot  here  state  any  law  accord- 
ing to  which  the  ratio  of  power  and  weight,  when  they 
balance  each  other,  can  be  found. 

What  familiar  instruments  act  on  the  principle  of 
the  wedge  ? — The  chisel  of  the  carpenter  is  a  wedge  ;  so 
is  the  blade  of  a  pocket-knife ;  each  having  a  sharp  edge, 
and  being  thicker  at  a  distance  from  it.  The  chisel  is 
usually  driven  by  blows;  the  knife  urged  by  pressure;  but 
the  cohesion  of  the  wood  is  the  resistance,  or  the  weight  to 
be  overcome  by  both.  All  cutting  and  piercing  instru- 
ments are  different  forms  of  the  wedge. 

Describe  the  screw. — The  screw  consists  of  a  cylinder 
having  a  spiral  groove  cut  around  its  circumference.  The 
small  screw  used  by  carpenters  for  joining  the  parts  of 
their  work,  on  a  small  scale  illustrates  this  arrangement. 
When  the  screw  is  to  be  used  as  a  machine,  the  cylinder  is 
made  larger,  sometimes  a  few  inches,  sometimes  several 
inches  in  diameter.  The  projecting  edges  left  between  the 
parts  of  the  spiral  groove  are  called  threads,  and  the  screw* 
works  through  an  opening  in  a  firm  block  having  a  spiral 
groove  cut  upon  its  interior  surface  into  which  these 


186  NATURAL  PHILOSOPHY. 

threads  just  fit.     This  block  is  called  the  nut,  or  sometimes 
the  concave  screw. 

The  top  ol  the  screw  is  called  its  head  :  it  is  the  part  to 
which  the  power  is  applied.  The  power  is  generally 
applied  by  means  of  a  lever  reaching  outward  from  the 
head. 

Explain  Fig.  99. — This  cut  represents  a  screw  set  in  a 
firm  framework,  as  it  is  often  used  when  a  great  pressure  is 
rig.  99.  to  be  exerted.     C  represents  tiie  head 

of  the  screw,  and  B  the  lever  by  which 
it  can  be  turned.  The  block  N  through 
which  the  screw  works  is  the  nut. 

AVhen  the  screw  is  turned  by  the 
lever  it  advances  through  the  nut  and 
pushes  the  movable  block  E  F  down 
upon  the  body  to  be  pressed.  An 
enormous  pressure  may,  in  this  way, 
be  exerted  upon  this  body. 

The  power  is  applied  at  B,  and  the  pressure  exerted  is 
the  weifjht. 

What  is  the  relation  between  the  power  and 
weight  ? — As  the  screw  is  turned  the  power  at  B  must 
travel  around  a  large  circle,  and  when  it  has  gone  once 
around,  the  screw  will  have  advanced  through  the  nut  a 
distance  a  c,  the  distance  betweeix  two  contiguous  parts  of 
the  thread.  Now  it  is  found  that  the  weight  will  be  as 
many  times  greater  than  the  power,  as  the  circumference 
through  which  the  power  travels  is  times  greater  than  this 
little  distance  a  c.  In  other  words,  the  law  may  be  stated  : 
"•  The  power  and  weight  will  balance,  when  the  power  is  to 
the  weight  as  the  distance  between  two  contiguous  threads 
is  to  the  circumference  in  which  the  power  moves.'' 

How  many  machines  have  now  been  described  ? — 
We  have  now  described  six  simple  machines  by  which 


MACHINERY.  l,s; 

weights  may  be  lifted  or  resistance  overcome.     Let  us  bring 
their  names  together. 

The  Lever.  The  Inclined  plane. 

"    Wheel  and  axle.  "   Wedge. 

"    Pulley.  "   Screw. 

There  are  no  others.  All  forms  of  machinery  are  made 
by  combining  these  six.  Just  as  when  you  learned  the 
twenty-six  letters  of  the  alphabet  you  possessed  all  the 
characters  used  in  the  many  thousands  of  words  of  oar 
language,  so,  having  learned  of  these  six  simple  machines, 
you  have  learned  of  all  the  elements  out  of  which  in- 
genious men  have  contrived  the  wonderful  variety  of 
mac  hinery  to  be  found  among  civilized  nations. 

What  one  principle  holds  good  in  the  action  of  all 
these  machines  ? — A  small  power  acting  swiftly  may 
put  a  large  weight  in  slow  motion  ;  or  a  great  power  act- 
ing slowly  may  put  a  small  weight  in  rapid  motion. 

This  law  holds  good  in  all  forms  of  machinery,  what  is 
lacking  in  force  must  be  made  up  in  velocity.  And  in 
just  this  lies  the  advantage  of  using  machinery.  It  helps 
man  to  transform  velocity  into  power  to  overcome  resist- 
ance. No  power  can  be  created  by  it,  and  wherever 
power  is  gained  it  is  bought  and.  paid  for  in  velocity. 

By  what  agents  is  power  to  move  machinery  ex- 
erted ? — Animals,  water,  wind,  and  steam  are  the  agents 
or  powers  most  commonly  applied  to  move  machinery. 
Men  or  horses,  by  means  of  pulleys  or  wheels,  may  bo 
seen  lifting  stones  where  large  buildings  are  being  erected. 
Saw-mills  and  flouring-mills  are  often  worked  by  u  water 
power"  acting  upon  the  circumference  of  large  wheels. 
Wind-mills  for  pumping  water  and  for  other  purposes  are 
turned  by  the  wind,  while  in  the  steam-engine  and  all  the 
vast  machinery  in  the  arts  moved  by  it,  the  motive  power 
is  steam. 


188  NATURAL  PHILOSOPHY. 

What  is  meant  by  the  term  "  Horse-Power  "  ?— Tl  o 
term  horse-power  is  used  in  estimating  the  effect  which  a 
steam-engine  or  other  machine  can  produce.  The  term 
has  no  reference  to  the  animal  whose,  name  is  used  :  it 
simply  means  a  certain  amount  of  work.  A  power  to  lift 
33,000  Ibs.  through  1  foot  in  1  minute  is  a  ¥'  horse-power.'' 
So  when  an  engine  is  described  as  an  engine  of  "'10  horse- 
power," we  understand  that  it  is  able  to  do  a  work  equal 
to  that  of  lifting  10  times  33,000  Ibs.  1  foot  in  1  minute. 

THE    STEAM-ENGINE. 

How  can  steam  be  applied  to  machinery  ?— The  ex- 
pansive power  of  steam  (p.  142)  was  known  lonj  before 
any  means  were  devised  to  use  it  in  machinery.  Animals 
could  be  harnessed  to  a  machine  easily  :  water  and  wind 
would  act  upon  wheels  and  turn  them,  but  how  could 
steam  be  harnessed  or  applied  ?  In  the  open  air  steam  is 
as  feeble  as  an  insect,  but  when  confined  in  a  close  vessel 
its  efforts  to  expand  produce  enormous  pressures.  If  it 
can  be  confined  and  yet  be  properly  brought  against  a 
machine,  the  machine  will  be  put  in  motion.  This  is 
accomplished  in  the  steam-engine. 

How  is  this  accomplished  ? — For  this  purpose  a  cylin- 
der is  provided,  having  a  piston  fitting  its  interior  nicely, 
and  able  to  move  smoothly  from  one  end  to  the  other  back 
and  forth.  Steam  is  made  to  enter  the  cylinder  first  at 
one  end  of  the  cylinder,  then  at  the  other,  and  the  pressure 
of  the  steam  behind  the  piston  knocks  it  back  and  forth 
from  end  to  end  with  enormous  force. 

.Explain  Fig.  100.— Fig.  100  will  illustrate  this  brief 
description.  C  represents  the  cylinder,  and  P  the  pis'on 
which  can  move  freely  from  end  to  end.  S  represents  the 
pipe  through  which  the  steam  comes  from  the  boiler.  It 
enters  the  box  shown  by  the  open  space,  ?/,  and  the  steam 


MACHINERY. 


189 


passes  from  there  in  a  direction  shown  by  the  upper  arrow, 

and  enters  the  cylinder  at  the  top.     Once  in  the  cylinder 

it  expands  against  the  piston   and   pushes  it  down,  the 

old    and    useless    steam   below,  Flg  J00 

passing  at  fie  same  time  through 

an  opening  at  the  bottom,  and 

thence  up  to  the  space  O,  and 

iway  through  a  pipe  not  shown 

in  the  cut. 

When  the   piston  has  almost 

reached  the  bottom  of  the  cylin- 
der, the  block  y  slides  up,  and 

covers  the  passage  leading  to  the 

top,  but  uncovers  the  one  leading 

to   the  bottom.      The   steam   is 

then  forced  through  the  lower 

passage   to   the    bottom   of    the 

cylinder,  and,  expanding  there, 

drives  the  piston  to  the  top  again- 
The  block  y  then  slides  down 
and  opens  the  upper  passage :  the 
steam  goes  to  the  top  behind  the  piston  and  drives  it 
down.  The  block  y  then  slides  up  again  and  opens  the 
lower  passage :  the  steam  goes  to  the  bottom  behind  the 
piston,  and  drives  it  to  the  top. 

You  see  that  by  the  slide  valve,  y,  the  steam  goes 
first  to  one  end  of  the  cylinder,  then  to  the  other,  and  thus 
drives  the  piston  P  back  and  forth. 

Now  this  piston  P  has  a  rod  firmly  fastened  to  it  which 
reaches  up  through  the  top  of  the  cylinder,  and  which  is 
pushed  out  and  drawn  in  by  the  moving  piston.  It  is 
called  the  "piston-rod."  The  outer  end  of  this  piston-rod 
acts  as  a  power  to  move  levers  or  turn  cranks,  and  thus 
give  motion  to  machinery. 

The  steam  moves  the  piston,  the  piston  moves  tlio  pis- 


190 


NATURAL  PHILOSOPHY. 


ton-rod,  and  the  piston-rod  gives  motion  to  the  machinery 
outside. 

Explain  Fig.  101. — The  picture,  Fig.  101,  shows  how 
the  piston-rod  gives  motion  to  inadmiery  in  one  form  of 


the  steam-engine. 


Fig.  101. 


''''•••     '  _         '    •     '  •'•'     •' •!" 


In  the  first  place,  at  the  left,  we  see  the  cylinder  with 
0110  side  cut  away,  sj  that  we  may  see  the  piston  (P)  inside. 
The  steam  is  supposed  to  be  entering  the  valve-box  at  S 
and  going  to  the  upper  part  of  the  cylinder,  pushing  the 
piston  down,  just  as  we  described  when  we  explained  Fig. 
100. 

The  piston-rod  A  D  is  fastened  to  one  end  of  the  large 
and  strong  lever  II  K.  As  the  piston  goes  down  it  pulls 
this  end  of  the  lever  down  and  throws  tlie  other  end  K,  up. 
When  the  piston  rises  in  the  cylinder  the  piston-rod  pushes 
the  lever  end  II,  up,  and  throws  the  other  end  K,  down. 


MACHINERY.  191 

Now  as  the  lever  at  K  goes  up  and  down,  it  pulls  and 
pushes  upon  the  strong  arm  J,  and  in  this  way  turns  the 
crank  C.  The  large  wheel  W  W,  fixed  upon  the  axle,  will 
thus  be  put  in  motion. 

Where  may  -we  find  engines  of  this  form  ? — This 
form  of  engine  is  often  used  on  steamboats.  The  great 
lever  II  K  may  be  seen  above  decks  moving  alternately  up 
and  down  when  the  steamer  is  in  motion.  It  is  sometimes 
called  the  "walking-beam."  The  strong  arm  J  reaches 
clown  into  the  boat  and  tarns  an  enormous  iron  axle,  which 
reaches  quite  through  the  boat  from  side  to  side,  and  has  a 
paddle-w/ieel  at  each  end. 

Is  motion  always  communicated  in  this  -way? — 
The  way  in  which  the  piston-rod  gives  motion  to  other 
parts  is  very  different  in  different  engines.  There  is  no 
"  walking-beam  "  in  a  locomotive,  you  know.  You  can  see 
in  Fig.  101  that  the  piston-rod  is  jointed  at  B.  Now  in 
the  locomotive  engine  the  outer  end  of  the  jointed  part  D 
is  fastened  directly  to  the  "drive-wheels'"  of  the  locomotive 
at  a  point  between  its  centre  and  its  circumference.  The 
cylinder  lies  horizontal,  and  as  the  rod  A  moves  back  and 
forth  the  "  drive- wheel "  is  turned  and  the  locomotive 
rolled  forward. 

Engines  used  for  other  purposes  than  those  just  mention- 
ed are  made Jn  different  forms.  A  visit  to  some  manu- 
factory, where  you  may  see  for  yourself  the  construction 
and  action  of  one  of  these  most  wonderful  machines,  will  do 
more  to  make  you  acquainted  with  its  parts  and  their  uses 
than  can  be  gained  from  books,  even  though  you  study 
long  and  faithfully. 

What  is  a  high-pressure  engine  ? — Sometimes  the 
steam,  after  pushing  the  piston,  is  made  to  escape  into  the 
air  in  puffs.  This  is  done  in  the  locomotive  engine,  and  it 
gives  rise  to  the  irregular  puffs  heard  especially  when  the 
engine  starts.  Now  this  steam  must  \>&  puihed  out  against 


192  NATURAL  PHILOSOPHY. 

the  air,  which  presses  with  a  force  of  15  Ibs.  to  the  square 
inch  to  keep  it  in  the  cylinder.  The  steam  which  moves 
the  piston  must  exert  force  enough  to  overcome  this  pres- 
sure before  it  can  exert  any  to  move  the  machinery  outside. 
Its  pressure  must  be  at  least  15  Ibs.  to  the  square  inch 
higher  than  the  machinery  would  otherwise  require.  On 
this  account  the  engine  is  called  a  high-pressure  engine. 

What  is  a  low-pressure  engine  ? — In  other  engines 
the  steam,  after  having  moved  the  piston,  is  allowed  to 
pass  through  a  pipe  into  a  closed  chamber,  where,  by  cold 
water,  it  is  condensed.  The  withdrawal  and  condensation 
of  the  steam  removes  the  pressure  from  in  front  of  the 
piston,  and  the  force  of  the  steam  behind  it  may  be  all 
expended  in  moving  the  machinery.  Since  the  steam 
has  less  work  to  do,  it  can  do  it  with  less  pressure  than  in 
the  high-pressure  engine,  and  this  form  is  on  this  account 
called  the  low-pressure  engine. 


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